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Chapter 7

The Feeling Principle Is Unextended

92. What has been said shows once more how serious a mistake it is to confuse the sensiferous principle, which causes sensation but does not itself feel, with the feeling principle, which feels but does not cause feeling. In other words, it is a mistake to confuse body with soul.

We are forced to conclude that life resides properly speaking in the feeling principle although the sensiferous principle, if considered as acting directly and constantly on the feeling principle, is called animate or living body, and if considered as acting only indirectly on the feeling principle, that is, modifying the animate body, is called brute body. Nevertheless in both cases the sensiferous principle simply remains what it is, and feels nothing. The life attributed to it does not have the same meaning as that attributed to the feeling principle, the proper seat of life. In the sensiferous principle, which is alive only in the sense that medicine is said to be healthy, we simply find a cause that stimulates feeling.

93. Having distinguished the two principles and the two series of phenomena so that one cannot be confused with the other, the philosopher now has to concentrate more explicitly on the relationship between these two principles and two series of phenomena.

In this and the following chapters, I want to offer only a few thoughts on this relationship to draw attention to the importance of such investigations. I shall begin by pointing to the opposing aspects in the relationship between these two principles, and show that the body, the principle of extrasubjective phenomena, is essentially extended, while the soul, the feeling principle and principle of subjective phenomena, is essentially unextended.

94. I take any body whatsoever as the object of my thought, and I ask myself what is its characteristic, essential property. Without doubt, the body that I conceive mentally (and I can speak only about my own mental conceptions, not of anything else) has the following essential, characteristic property: every single part is outside every other part. And the same can be said about every other body. I conclude therefore: `Body is an entity whose essence is such that every part, great or small, which can be thought in it, is outside every other part'.

95. Having established this essential property of body, I now wish to see if it is possible, or impossible and contradictory, for the body to be the feeling principle. The core of the difficulty can be expressed as follows. Let us suppose that feeling adheres to a body as a property of the body. In this case, all the parts I assign in the given body will possess feeling. However, each part in the body is essentially outside every other part. Because each part has its own boundaries, therefore, our supposition necessarily requires that the feeling adhering to these parts likewise remains within these boundaries and limited to the individual parts of the body. Consequently, the feeling of one part will be outside the feeling of all the other parts. If this were the case, the feeling would make up not a single feeling principle, but as many principles as there are assignable parts in the body. Moreover, each of these feeling principles would feel only its own part without extending to the other parts because, as we said, each part to which a feeling principle adheres is outside all the other parts.

Now it is also certain that the parts thought and noted in a body can be divided indefinitely, and that each of the tiny assignable sections which make up the whole body is always outside every other. The feeling of each of the minute sections, therefore, will be outside the feeling of all the others. But however small we imagine a part to be, it can always be mentally divided into still smaller sections. Thus we can go on assigning feeling principles without ever arriving at the last feeling principle. Finally, we can choose between two suppositions: either the minute parts remain extended, and therefore with parts always allocated outside one another; or (and this supposition is absurd as we have shown)(53) the minute extended parts finally come to be changed through constant division into simple points. In the second case, feeling would adhere to simple points and would never be suitable for feeling that which is extended; in the first case, the true seat of feeling would never be discovered and the final consequence would be that of the second case. Because the minute sections would continue to be extended, with each of their assignable parts outside every other, the feeling adhering to each point could never emerge from that point (a point never exceeds itself) and therefore would not be suitable for feeling what is extended.

96. This argument provides the evidence for a very beautiful, unassailable truth: `That which is extended cannot feel what is extended.' What is extended always has its parts outside each other - one part is not extended inside another. If one part feels, its feeling would, therefore, be outside the feeling of the other parts, and any feeling with an assignable point in the part would be outside the feeling of all the other points.

97. It is clear that extension, as an essential property of body, makes the body different in its essential properties from the essential properties of feeling, which cannot be found in what is extended but only in a completely simple subject to which the whole of what is extended is simultaneously present. There is, I think, no answer to this demonstration of the simplicity of the feeling principle, if the demonstration is understood correctly.

98. An important corollary dependent upon this demonstration concerns the correct concept we should form of the simplicity of the feeling soul. We should never imagine the soul as a mathematical point (as some do). If the soul were a mathematical point, the feeling proper to this point could never go outside the point (for the reason we have stated) and would not be suitable for feeling anything extended.

99. Moreover, a mathematical point is only a mental being which we form by abstraction. It never exists in nature or of itself. As the extremity of a line, it cannot be separated from the line. If it were separated it would simply form another line, however small, but never a point. The concept of a point is not that of any real being, but of a simple relationship, that is, of the term of linear extension. Those who take a mathematical point as an example of the simplicity of the soul set out on a path that leads them away from the right understanding of the feeling principle.

100. We have to be on our guard against seeking the nature of this feeling principle through images and in abstractions taken from matter. We need to be satisfied with observing the feeling principle in itself and in its proper phenomena without adding anything extraneous to the principle. If we do this, we can convince ourselves, notwithstanding the wonder we experience, of the existence in nature of a being different from material beings, and naturally distinct from them. This being has no likeness whatsoever to matter, although it can be related actively and passively with matter. Penetrating thinkers soon find their wonder directed at a new object, and begin to notice that it is more difficult to explain the existence of matter than that of an immaterial principle possessing a mode of being altogether different from material being.

101. Another very important corollary from what we have said concerns the proper seat of the soul in the body. If the soul is neither extended nor simply a mathematical point, where is it to be found?

As long as the soul is regarded as a point, it is natural to ask about its location. A point always has a situation relative to matter, and this place can be mathematically determined through three planes or three lines in different planes. But if the soul is not a point, does the question about the place of the soul in the body have any meaning?

102. As soon as we have formed a correct concept of the simplicity of the soul, it becomes clear that the question has no meaning in itself. After the appearance of philosophy, however, the problem occupied and divided astute minds for many centuries. No compromise could be reached on an answer because in fact the question permitted no answer. Descartes decided that the soul was to be found in the pineal gland; Lancisi, De la Peyronie, Teichmeyer placed it in the corpus callosum. Digby, not having found it in the corpus callosum, decided that its dwelling was the septum lucidum. After a vain search here, Drelincourt settled for the cerebellum, Willis for the corpora striata, Le-Cat for the meninges and Vieussens for the marrow part of the brain called the centrum ovale. Others located it, and go on locating it, wherever they please. But such learned discussions and work, the fruit of an unclear idea and a confused concept of the soul, are useless despite the multiple theories, systems and stupendous libraries constructed upon them.

103. Once the notion proper to the simplicity of the soul has been adequately clarified, great theories vanish like smoke. Once we realise that the soul is not extended, and is not a mathematical point, it is easy to conclude that it does not occupy space, and is not to be found in one place rather than another. We do indeed discover traces and effects of its action in different parts of the body, but the soul itself is not found in any large or small part of the body, nor in the body as a whole, nor in any point. Its mode of being cannot be compared, proportioned or likened to anything material, or to any property of matter. As we said, it possesses only a relationship of activity and passivity to matter, which is nothing more than a relationship of feeling.

104. We need to demonstrate the same truth in another way. This proof also starts from facts.

It is very easy to see that many parts of the human body used immediately and obviously in the service of sensations are duplicated, while those which support organic life are single. The stomach, heart, intestines, spleen, liver, and so on, form unique working parts in the body; the eyes, ears, nostrils(54) are duplicated, and the nerves receiving the impressions - the optic, acoustic, taste and olfactory systems - are distributed in symmetrical pairs. In the case of the exterior senses, therefore, each person has a twofold impression through his two eyes, ears and so on. Moreover, each impression taken on its own has a corresponding sensation. If I close one eye, the other gives me a complete sensation; if I block one ear, I receive all the sound from the other, and so on. When I use both eyes or ears together, however, a single sensation corresponds to the two impressions. My two eyes do not provide me with two sight-objects: my two ears make me hear the single flute that is playing, not two. This is a fact, and with this as my basis I reason as follows.

105. Suppositions about the fusion in the brain(55) of the two impressions received, for example, in the optic nerves are not confirmed by anatomical findings which show rather that the brain itself is divided into two distinct parts.(56) This is not, of course, a decisive argument. The conjunction between each pair of corresponding nerves could yet be found, or could be present in nature without its being found. But there are other effective reasons for excluding it. If in fact the two optic nerves were united at some point, how could the two eyes see different images simultaneously, as sometimes happens? In such a case, the same small bundle of nerves in which the two optic nerves united would be affected differently by either optic nerve. The result would be a single, in-between movement which would be neither the one image nor the other, but a confused sensation made up of both.

106. Moreover, an obvious proof that the two nerves cannot unite to form one is given by the following reasoning.

The action of light on the two small, end-surfaces of the optic nerves is not uniform unless the object seen possesses a uniform shape and colour. The action varies with the variations presented by the shape and colour of the seen object. If it were possible to distinguish physical points or tiny surfaces in the two surfaces formed by the iris, then the light, in accordance with the tint that has to be aroused in the sensation, would make a different impression in each tiny surface or section. The image or total sensation aroused is lined and varied according to the various actions of the light, and results from the complex action of the light on all the tiny parts or sections of the terminal surface of the optic nerve. Hence every varying point of colour or tint in the image or sensation corresponds to a point in the extremity of the optic nerve affected variously by the light.

We can now conceive mentally either that the optic nerve is formed of a tiny cable of minute nerve-threads, each of which is responsible for transporting to the brain the impression made upon it by a little ray of light (and this seems certain); or that, in place of these nerve-threads, the optic nerve transmits every differing point of impression through distinct movements. In this case, the nerve itself is moved differently along the length of each of its sections as though it were made up of many, very subtle, smaller nerves, each with its own movement. In both hypotheses, I maintain it is easy to prove the impossibility of the optic nerves terminating in a single, united nerve which gives rise to a single image.

If a single nerve hypothesis were possible, the phenomenon could only be explained by admitting that two corresponding filaments of the optic nerves come together and continue as a single filament, just as the nerve which receives the two filaments would continue as a single nerve. Otherwise, if the filament of one optic nerve attached itself to a filament of the central nerve and the filament of the other optic nerve attached itself to another filament, the corresponding impressions and sensations would have to be two, not one. But granted that every nerve filament of one eye which transmits a point of the impression (corresponding to a point of the image or sensation) is united to the same point of the central nerve as the corresponding nerve filament of the other eye, the unity of the image could no longer be explained, because such a unity could come about only if there were a unity of impression in the central nerve. But this unity would require that the filament of the nerve of one eye which carries the impression, for example, of a tiny red square, should bring this impression of red to the point in which the corresponding filament of the other eye brings an equal impression. If the red square were brought to the central nerve not by the corresponding filament, but by another joining the central nerve in a different point, there would be two, not one, red squares in the central nerve. If the corresponding filament brought to the central nerve the impression of a blue square, two impressions would be concentrated in the same point of the central nerve, one corresponding to the red and one to the blue sensation. The result would be an in-between sensation in which the image would be confused or made up of alternating and varying colours. The object viewed by both eyes would be coloured differently from the same object seen with one eye.

In order that the two corresponding filaments may transmit the same impression, they would have to be touched at their extremities by an equal action of light. This, however, is precisely what does not happen, as we can easily notice by observing how light coming from the same object strikes the two eyes. It does not strike the two eyes in such a way as to make an impression with the same rays on the same extremities of the optic nerve, nor does it strike the eyes from the same angle, especially if the eyes are turned to look at something to one side. If this object has a white spot, we still see it as a single spot with both eyes although the light moving from that spot does not fall exactly upon the same place in the retina of both eyes, nor affect the same nerve filaments which are supposed to unite in a single point of the central nerve. On the contrary, it strikes the filaments that join the central nerve in different places. According to such a hypothesis the white spot should be duplicated because a twofold impression has been brought to the supposed central nerve that receives the two optic nerves.

Unless I am mistaken, this is an irrefutable, convincing proof that even if the two optic nerves did join to form a single nerve, the unity of sensation corresponding to equal impressions in the two eyes would still not be explained.

The same proof can be strengthened if we note the optical law according to which two impressions present a single object to our feeling. `When two optic axes terminate in a single point, this point is seen as something simple, not double.' The direction of the optical axes depends upon the terminal plane of the optic nerve against the light. The compenetration of the two impressions, therefore, does not arise because the extremities of the nerves are affected, but from the angle at which the light falls upon these extremities. It would be of no help if the nerves were joined at some point in the brain. Everything depends upon the position of the plane which the nerves present to the light at their extremities.

107. G. B. Venturi affirms that there would be grounds for believing in the union of the impressions of the two eyes in the same part of the brain if it were true, as some have maintained, that a green sensation resulted from blue rays being made to fall on one eye and yellow rays on another. Venturi himself does not give credence to the union because he has never managed to reproduce the green phenomenon.(57) But even if green were reproduced, such a phenomenon would not provide sufficient proof for accepting the union of the two impressions: the unification of the image does not arise from intermingling of colours, but from identification of space.

My own opinion is that even if the supposed phenomenon were verified, it would serve to confirm the separation of the two optic nerves rather than their union in a single nerve. If I look with one eye at a patch of blue and with the other at a patch of yellow, it is certain that my yellow and blue sensations are distinct as long as the optical axes remain parallel. This alone is sufficient to prove that the two optic nerves do not form a single nerve in which the impressions are centred in the same space. But whenever I have crossed my eyes, I have always seen one colour on top of another, while both become lighter and as it were transparent. This proves that the image is not unified through the conjunction of the optic nerves, but through the identical space to which the soul refers the two images. This space is the proper term of the feeling principle, and hence internal, relative to the soul. I have also observed that I could render colours more or less strong or weak at will by looking more steadily with one or other eye. Looking equally steadily with both eyes, however, I found the colours taking on a paler hue and fading into one another. All this shows that their union does not come about materially in a common nerve.

108. The union of the two nerves in a single nerve cannot be granted therefore, or if it is granted it does not help to concentrate the impressions. Truly distinct impressions remain in the body, and a single sensation in the soul which, in this case, is like the single sound from a single bell rung by two people. Such a sound is obviously a phenomenon that does not occur in the body, although the impressions that go to producing it do occur there. If this phenomenon adhered to the bodily impressions without being distinct from them, it would be twofold as they are twofold. It is therefore distinct and separate from the bodily impressions and pertains to another principle bereft of bodily multiplicity. This principle is called the soul.

109. I realise that this demonstration, although it appears very convincing to me, will cause difficulties to some readers. I hope that the following reflections will resolve their problems.

First, we have to eliminate the idea, held by some, that two sensations as well as two impressions of every object we see are truly in our eyes. It would seem, according to this objection, that the two sensations appear as one because of touch, which certifies that only one object is actually seen. Our judgment, the objection continues, habitually goes beyond the twofold sensation to the simple tactile object to which it refers the two sensations of the eyes.

110. A careful examination of the facts presents the following reasons for not accepting this explanation.

1st. While the objection is applicable to the eyes, it cannot be applied to the ears because the sound-sensation is not referred to the sonorous body in the way that colour is referred to the tactile body.

111. 2nd. If we did in fact have two sensations, it would be impossible not to be aware of them, at least through determined, concentrated effort. Sensations remain in us, whatever our habitual judgments. There never will be a case in which sensations, especially ocular sensations, which are very noticeable, will disappear completely from our attention in virtue of some habitual judgment we have made. This becomes even more marked when we consider that the judgment made about the unity of the object is different from that made about sensations. If the sensations were two, and the tactile object one, we would pronounce two judgments, one of which would affirm the two sensations, and the other a single object indicated by the two sensations.

Let us consider a simple fact which will show us without doubt that we could not avoid observing the two sensations if they were present in us. Sometimes our eyes become distorted if, for instance, we bring a small irregular object very close to them. In this case two sensations are indeed present. Moreover we are immediately aware of them and of what appears to be two objects. But with hand and judgment we correct the mistake, and conclude that they are one. The same thing happens in other cases where our judgment corrects the error to which our feeling-power would lead us. If we stand at the end of a long drive of cypresses, for example, we judge that the trees are all more or less of the same size. This judgment, however, does not destroy or even change the actual sensation we have of the trees, nor does it prevent us from becoming aware of the sensations. We are still capable of telling ourselves that we see the last cypresses in the drive as much smaller than the first. Our habitual judgment does not prevent us from noticing the sensations although they have been emended, as we say, by the judgments themselves.

112. 3rd. If naturally we had two sensations in our eyes and corrected this error through habit, it should be possible to discover the twofold sensation in babies. A habit as strong as this, which enables us to judge the opposite of what sensations show us and would lead us to nullify a sensation (this is what is implied), can be formed only gradually. There is no experience of this in babies, however.

113. 4th. Moreover, we need to consider that the sensation received by a person with both eyes open and motionless (we speak only about sight because many of the things said about this sense can be applied to other senses)(58) is a single sensation made up of three sections. In other words, the whole luminous orbit seen by the two eyes can be divided into three parts: 1st. the right part, not seen by the left eye; 2nd. the left part, not seen by the right eye; 3rd. the middle part, seen by both eyes. The two impressions produce a single sensation only in the middle part where they become identical. Despite the different view of these three parts of the visual orbit when seen by one or two eyes, the orbit itself presents a uniform, continuous and therefore single sensation where no lines of separation can be found. Consideration of this phenomenon enables us to conclude that the habitual judgment could not simplify the sensation, if this were a double sensation, because it would simplify the total sensations received by the eyes, not just a part of them. The fact is, however, that one part of the sensation of each eye remains separate, and the other part coalesces. There must therefore be a difference not only in the judgment passed on the same sensation but also in the nature itself of the sensation. Consequently, the middle part of the visual orbit has indeed two impressions in the eyes, to which corresponds, however, a single sensation in the soul. On the other hand, the extreme parts of the orbit have two impressions to which correspond two sensations in the soul. These sensations are in continuity with the sensation of the middle part in such a way that we see a single visual screen without divisions. This is possible because the space of the screen, which adheres to the soul, is one.

114. 5th. Finally, the matter can be reduced to a rigorous demonstration. It is clear that when we see a single body, e.g. an orange, with both eyes, the orange as seen proffers a single space, a single round disc. If there are two sensations they have only the single disc as their term because what we feel is certainly reduced to that disc. If, however, the same disc is the term of both sensations, the sensations can only be one sensation because every sensation essentially possesses extension. The single extension, however, is not that of the real body because the real body is outside our eyes and soul. Nor can the extension be that of the impressions of light upon the retina because they would then be two. The soul, therefore, has a single space in which, under certain conditions and dependent upon certain laws, it arouses a single sensation on occasion of the twofold impression it receives in the optic nerves. The soul could not do this if it were not simple.

115. We now have to indicate the reason why the sensation corresponding to the two impressions of symmetrical organs is single in certain cases but duplicated in others. The law states: `Everything seen by the soul by both eyes is a surface plane with a position given by nature,(59) that is, a felt plane. If the two impressions occupy different parts of this plane, the soul has two sensations dependent upon different extensions and sometimes different colours; if the two impressions occupy the same parts of the felt plane, the soul has only one sensation relative to extension, although the impressions may be separated from one another in external (tactile) space. If the colours in the same space are different, the soul sees them mixed and, as it were, transparent' (cf. 107).

116. To understand this stupendous law, 1. we have to set aside the common opinion that sight of itself sees distances. It sees distances only in the way that distances are seen on walls painted with scenes in perspective. Perspective drawing presents distances to the imagination, not to sight. The eye, therefore, sees only a plane painted in different colours. 2. We have to notice the difference between touch and sight sensations. The former are curved surfaces because their locality is the surface itself of our body which varies in curvature. To these sensations are added the traces of motion that remain, dependent upon certain laws, in our feeling. Sight sensation on the contrary is always a perfectly flat surface in a position given by nature. Because touch sensations are distributed in different planes, they can be felt separately from one another; but relative to the eye, impressions made in different planes give sensations in a single plane. On the other hand, two impressions are often mixed because they appear on the same part of this given plane.

117. Sensations therefore which come from two impressions made on different planes identify when they fall on the same points of the plane destined by nature for sensation.(60) This identification of sensation is not the result of a lack of a feelable effect in the soul corresponding to the impression made on each eye. On the contrary, it is certain that a sensation corresponds in the soul to each optic impression; as we said, a single impression is sufficient to produce an entire, distinct sensation. But the law consists in this: the two effects, the two sensations, when brought into the soul by the two impressions in accordance with the condition we have described, are identical just as the sound of a bell rung by two arms rather than one is identical. And this explains why the identical sensation resulting from the two impressions which correspond to the same points on the feelable plane is somewhat more vivid when produced by two impressions rather than one - just as a bell has a louder ring when pulled with two arms.

118. Granted this law, the contrary is also true. That is: `If sensations correspond in different parts of the feelable plane (in what I call the optic plane) to the two impressions, they do not identify but remain separate.'

119. This explains 1. how the total sensation, received when both our eyes are open and motionless, is composed of the three parts we have indicated, but in such a way that only the middle part of the two sensations is identical. This happens because the impressions received by the two eyes from coloured bodies do not strike the same parts of the optic plane except in the middle where alone the sensations are identical; 2. how by distorting an eye we can sometimes have two clear sensations of a single object. This happens because the impressions received by the two eyes correspond to sensations in different parts of the plane of sensation, that is, the optic plane, so that these sensations cannot identify.

120. Finally, I note that the difference of the impression, considered relative to the sensation corresponding to it, is due to 1. either the difference of position in respect of the constitutive parts of the visual orbit or the difference of the size of the visual orbit; or 2, if the respective position of the parts of the visual orbit is equal, to the different power and perfection of the optic nerve.

121. If only the respective position of the parts and the size of the visual orbit is unequal, two clear, distinct images can be received. These are in fact the two extreme parts of the visual orbit, each of which is seen by one eye alone because the visual orbit of each eye extends further on one side than the visual orbit of the other eye. And these images are not confused. The same happens to animals which use single, independent eyes. Finally, when one eye is moved, the objects are duplicated because they change their position in the visual orbit and in the optic plane, and are seen as clear and distinct. In this last case, it sometimes happens that we say we see badly although our defective sight does not, properly speaking, reside in the sensations but in the error of judgment to which we are drawn by the sensations when we maintain that the object is twofold, not single. Such sensations are not defective or muddled as sensations, but as signs serving as a basis for judgment.(61)

122. If the light causing the image varies in force or precision, either because one organ is weaker than another or because one eye is looking through a lens, we then see the same image with one eye as with the other, but coloured differently. In this case, the confusion arising in the feeling subject renders the single sensation defective. We must note, however, that if one image is larger than another, as happens when a lens is used for one eye, the image is seen clearly from the point where its greater extension exceeds the bounds of the smaller extension; the smaller image, the nucleus as it were, while vividly coloured, is less distinct in its coloured sections(62)

123. We can now deduce a new proof of the simplicity of the feeling principle from its capacity to make one sensation from several sensations, even when they are varied. This would be impossible if the feeling principle were not one and simple. A few experiments will help to elucidate this.

Let us imagine that we are looking at a huge blue area without variation in colour. In our hypothesis, the colour fills our total visual field, but we view it with one eye weaker than the other, or with one eye strengthened by the use of a lens. As a result, one eye sees the blue less vividly than the other. If, however, we open both eyes together, the soul has a single sensation in which it sees the more vivid blue because, in forming a single sensation from the two it receives, the soul must have the brighter colour in the total sensation. If a colour of a certain tint is superimposed on another of the same tint, the colour and the tint remain the same; but if a colour in a strong tint is superimposed upon the same colour in a weaker tint, the brighter colour prevails. This shows that the soul, by integrating what it feels, produces for itself a single total sensation, and that confused sight results only when the many colours, varying in distribution, do not strike the same lines of sight.

124. Another experiment will show how the soul makes one sensation from two, relative to the extension of the sensation (the previous experiment demonstrated the unifying power of the soul relative to the brightness of the colour or light we see).

Let us imagine that we are looking at two trees in line, but standing slightly to the right of them so that when we close our right eye, we see the right edge of the nearer trunk just making contact with the left edge of the more distant trunk. The union of the two trunks appears as a single continual sensation of bark and trunk without any intervening gap.

If we now close the left and open the right eye, without changing position, we see the two trees with a space intervening between them. This occurs because the right eye, seeing the outline of the first tree from a different angle, separates the trees and restores each of them to its original size. The sensations given by the two eyes separately, therefore, differ notably, and their objects are distributed in different fields of sight.

How does the soul join these sensations in such a way that no confusion arises between them? What total sensation do we have if we keep both eyes open? If the soul simply joins the two sensations, it will duplicate the objects. If it superimposes one upon the other, it will confuse the sensations and once more duplicate the objects: if one eye sees an area as large as the two trunks together, and the other eye sees two separate trunks, each smaller than the double-trunk sensation and with a gap between them, the two sensations when superimposed ought to present one plane composed first of the two trunks united (the sensation from the left eye), then of what remains of the empty section, and finally of the other trunk (the sensation from the right eye). But the soul does not do this. Instead it retains the entire sensation of the right eye and, preserving the disposition of the parts given by this eye, uses the sensation of the left eye only to make the coloured area brighter.

This is explained by what we have already said: the two trees are perceived equally by the two eyes but retained by the soul without being duplicated. One eye, however, perceives in addition a gap between the trees and contributes this to the rest of the sensation so that the trees are at some distance from one another. This proves that the sensation itself of each tree is not tied to a place (this must be noted carefully), but exists in itself. In other words, the relationship of place between the things we see is given only by the sensation and therefore by the optic plane (cf. 118) with which the soul is furnished.(63)

125. Let us try a third experiment. In the previous example, we saw the two contiguous tree trunks in a single sensation, seen by one eye; then we saw them apart, with a space between them, as a result of their being seen by the other eye. This time we shall see the opposite: two sensations at a distance from one another will form a single sensation by the exclusion of the gap between them.

Let us take a small disc of any colour. We first place the extended palm of our hand sideways on between our two eyes to separate one eye from the other, and then bring the disc close enough to touch, or nearly touch the outer edge of the palm. One eye will see one part of the disc, the other eye the other part. The gap in between will be covered by the hand. Each eye, therefore, sees a part of the disc, without the parts being contiguous - they are divided by the thickness of the hand. If we now cross our eyes so that the lines of sight meet, the two visible parts come together and produce a single sensation which, however, is oval, not round: the two parts of the circle have united perfectly, top and bottom. The philosophical explanation of this phenomenon is to be found in the law already presented (cf. 115-118);(64) we need take the matter no further here. Observing the phenomenon will be sufficient.

126. It is clear, therefore, that these composite phenomena could not take place if the soul, which produces one sensation out of two, were not perfectly one and simple. Identification and composition of sensation by the soul are facts which cannot be explained in any way unless we accept the soul as perfectly simple. These facts offer irrefutable proofs of the simplicity of the soul.

127. Finally, I cannot conclude this chapter without drawing attention to another observation about the phenomenon of a double impression with a single sensation. The observation will be of considerable assistance not only by confirming the simplicity of the feeling principle, but also in explaining further the phenomenon under discussion, and forming a correct concept of the simplicity of the soul.

Let us imagine that the composition of the impression received in one eye (corresponding to the optic sensation) is the same as that received in the other, that is, equally divided so that every point struck by a colour and tint of light is at an equal distance from other points in each impression. In this case, when we close one eye, the other will have as a total sensation a coloured picture in which all the colours and their gradations will be distributed in the same proportions. For example, if the picture contains the image of a tree, a column and a human being, these three objects will have the same respective positions whichever eye they are seen by. The same may be said about the lesser divisions making up the tree, the column, the human being and the entire field of the coloured picture or screen formed by the sensation in each eye.

We can now make two suppositions. First, that the feeling principle is simple and totally devoid of parts and extension; second, that the feeling principle is extended. What would be the result of each of these suppositions?

If the feeling principle which receives an equal sensation from each eye is simple, it follows necessarily that the two visual sensations, occupying an equal position (in the optic plane), identify as one. The opposite could happen only if the feeling principle were extended. I am persuaded of this by the following argument.

If it ever happened that the two equal impressions made by the light in the two optic nerves aroused two sensations in the soul (which we take as synonymous with the feeling principle), the soul would see simultaneously two coloured orbits or screens in each of which it would see a tree, a column, a human being, and so on for all the objects and parts in the two screens which constitute the two sensations. As a result, there would be a space between the two trees, columns and human beings (in visual sensation objects are distinguished from one another only by being seen in different places; if one were seen in the same place and point as the other, it would already be identified with and absorbed by the other). But the distance between the duplicated objects could be determined only by the distance intervening between the two orbits in which the sensations appeared to the soul. What principle would determine the separation or mutual exclusion of these two orbits? What principle could establish their continuity or the gap between them? If one object is contiguous to the other, so that the two together form a single double-sized orbit providing a visual sensation with double width, we have to say that this phenomenon exists in the soul itself, that is, where the sensation is situated.

The visual sensation, we must carefully note, is in the soul. If therefore the two sensations in the soul, as well as the two impressions in the material organs, are distinct, there must be something in the soul holding the sensations apart in such a way that one visual orbit is in one part of the soul, and the other in another. But in this case, the soul has parts, which is against our supposition.

The argument is even clearer if two orbits appear in the soul divided by a gap. In this case, the interval, which is not furnished by either of the orbits, must be posited by the soul. That is, there must be in the soul which receives the impressions the interval with which the soul sees the separated impressions.

But the argument is equally strong whether the interval between the orbits is large, or small or non-existent. If the soul were to see the two visual orbits, there would never be a satisfactory explanation why these should appear separated from or contiguous to one another (one will always be outside the other wherever they are) except the very nature of the soul which would place the two orbits where they are rather than elsewhere. In other words, it would receive one orbit in one part of itself, and the other in another.

This is the conclusion we would have to reach if we supposed that the soul were something material. It is clear that if the soul were only the internal extremity of the nerves, the distance of the extremities in which the sensation arose would determine the separation of one sensation from the other. The same conclusion would prevail if the soul received the two impressions in two separate particles of itself. These particles or molecules would be outside one another, and consequently the sensations in each of them would be outside one another. The respective distance between the particles would also determine the respective distance of the sensations. But even in this hypothesis, which would explain how the two visual sensations remained double, there would be two insurmountable obstacles: 1. the unity of the feeling principle (because the molecule which felt one sensation would not be the molecule which felt the other sensation); 2. the impossibility of either of the particles feeling the distance or separation between each other (because they would be outside one another, and outside the distance separating them).

Our argument may now be simplified as follows: the visual orbit, in order to appear double in the soul, would have to present the soul with a space at least twice the size of each orbit. But this double space is found in neither orbit. In this case, the soul has to posit it of itself and receive the sensations in different parts if the sensations are to remain separate. If the soul has no parts, however, it is clear that the two sensations aroused in it, in the same point as it were, must compenetrate one another.

128. It will be objected that in each visual sensation the soul can distinguish and separate individual parts without our having to infer that the soul itself has parts. Equally the two sensations can appear distinct to the soul without our needing to posit parts in the soul.

But the cases are different, not analogous, and we have to distinguish between two quite opposite questions: `How does the simple soul feel the extension which is given it in sensation?' and `How does the soul feel an extension which is not given by sensation?'

All doubt about the first question is removed by demonstrating, as we have done, that what is extended can never be perceived and felt by what is extended. What is extended can be felt only by what is simple. The demonstration does not resolve entirely the mystery of what takes place in this particular case, and we shall have to speak about this at length. However, it is sufficient to convince us that there is nothing absurd in supposing that what is simple feels and perceives what is extended, although it would be absurd to suppose that what is extended would feel what is extended (cf. 94-96).

The other question `How does the soul feel an extension which is not given it by sensation?' admits of only one answer.The soul itself not only feels and perceives what is extended but feels and perceives it as extended. In other words, the mode itself of perceiving and feeling what is extended is subject to the laws of extension or (and this amounts to the same thing) the act of the feeling soul is the act of an extended being.(65)

129. Careful consideration shows that the extension accompanying visual sensations is provided by the visual sensations themselves; the extension necessary to divide and separate them is not. It is not easy to understand what I want to say, but it can be grasped if thought about attentively. The picture which I see is one thing; where I locate this picture is another. Although the same picture is transmitted to me by both eyes, the eyes never indicate to the soul the space in which it has to locate the two pictures. Each eye accomplishes its function by giving the soul its picture; each picture exists in itself, not in any place, and has no relationship with the other picture. When the soul receives a picture from one eye, and an equal picture from the other, the soul itself has to locate them according to its own nature. If it is simple, it will not provide any place for them, nor locate them anywhere, but retain them simply as they are given to it. In this case, they will identify with one another as the soul sees the same space in each equal sensation (the place in which the two sensations would have to be located would be a space or extension added by the soul because the local relationship of the two whole sensations is found neither in one nor the other, but only in the subject which perceives them both). The soul, therefore, feels the sensations outside place, in themselves alone. It retains, that is, sees the space they have in themselves, but does not locate this space in another space. As a result the two spaces, not located in different places, become of necessity one single space.

130. As we have already noted, philosophers were led into many errors about sensation through paying almost exclusive attention to the phenomena of sight, the properties of which they adapted to all other senses. Another, perhaps greater source of error was the incapacity of the majority of philosophers to distinguish between the two series of phenomena that we have noted.

As a result, they confused subjective with extrasubjective phenomena (cf. 84-91), and failed to see the wonder present in the origin of sensation, although they thought they had found the explanation of such a stupendous phenomenon. The following is one example of the way they conceived the arousal of visual sensations. `Light depicts tiny images on the retina of the eye, which is only the optic nerve itself spread over an extremely soft, white film of skin and therefore capable of receiving all the various tints presented by colours. These little images, depicted with such delicacy on the nerve-membrane, are transmitted through the optic nerve to the brain where the soul receives and feels them.' This is well put, but how do these philosophers know that light depicts the tiny images on the retina of the eye? They appeal to observation, and refer to the state of the eye when it is removed from the cranium of an animal immediately after the animal's death.

If we take away the covering from the retina, they say, and then point the eye like a lens towards a well-lit object, with the pupil turned towards the object and the reverse side towards ourselves, we will see the back of the stripped retina reproduce faithfully, but inverted, the tiny image of the object.

It is impossible to be satisfied with such a badly applied experiment. It is undeniable today (and was well-known to antiquity)(66) that secondary corporeal qualities, such as colours, do not adhere to bodies as they appear to, but are merely sensations belonging to the subject which perceives with the senses the action of bodies.

The image which I see in the eye of the dead animal is, therefore, quite different from the visual sensation of the image, and cannot explain sight in a living being. Even if I could see into the retina of a living being, I would not see colours truly existing in that retina. These would be only my own sensation, which is in me. In the retina I am looking at, there is only a force causing my perceived colours, but not an image made up of variously distributed colours, variously distributed sensations. This observation alone is sufficient to destroy the explanation attempted by the philosophers we have referred to: it totally refutes the possibility that light depicts something in the retina that can be transmitted to the brain. Granted the presence of colour, granted some image, sensation is already present, and there would be no further need of any kind of mechanism or transfer to explain the origin of sensation in such a case. But the fact is that the tiny image supposed to be visible in the retina of some other person is not that which he actually sees. He sees something totally different, for example, an immense scene terminated by a horizon of hills or sea. The image visible in his eye is only a tiny circular patch, scarcely a hair's breadth in diameter. If I could see in the living being the image depicted on his retina, I would see only a speck of black where he sees an entire world. My sensation, therefore, is not his. My sensation is the tiny image that I see, but he does not. His sensation is what his own soul sees. If we wish to discover the origin of sight sensation, therefore, we have to abandon altogether the simplistic arguments of the philosophers we have mentioned.

But if it is not true that light carries the colours on to the retina as an artist puts colours on a canvas(67) (and no educated person can disagree with this today), the phenomenon of colour-sensation still remains intact and unexplained. Our question `How is sensation aroused in the soul?' still has to be faced.

131. The force of the question will be felt more strongly if we try to discover what the action of light actually does in the iris. Granted that it does not put colours there (the colours are the sensation which arises as a result of the action of the light, and are not therefore the action itself), we can only think of tiny impacts and displacements that the light induces in the optic nerves either through mechanical impulse, or chemical affinity, or some organic law.

It is also certain that there is an obvious analogy between the auricular and visual sensiferous phenomena. Sound is produced in the soul only if the means - the air, say - that strikes our acoustic nerve does so repeatedly, causing tremors of a certain frequency. The deepest sound that can be heard by human ears is produced by a frequency of thirty-two air waves per second from the oscillations of the sonorous body; the highest range of sound requires oscillations and waves to the marvellous number of 16,284 per second. Between these two extreme numbers, we have the number of waves necessary to arouse intermediate sounds. It is clear, therefore, that the movements caused by such action of the air in the acoustic nerve are only vibrations of certain frequencies. A certain degree of frequency and speed is necessary in these vibrations which move the acoustic nerve if the soul is to respond with a sound-sensation. No sound will be occasioned in the soul if the frequency and speed of the air waves is insufficient to cause less than thirty-two tremors per second in the nerve.

The following is a probable explanation, as far as I can judge, of the way in which similar movements are required in the optic nerves in order to arouse colour sensation. The mechanism needed can, I think, be conceived like this.

Imagine that the optic nerve, as I said before, is divided into innumerable, very thin threads, each of which terminates in the retina. The tiny nerve heads, packed together on the external surface of the retina,(68) are struck by rays of light falling upon them. These rays are like the fingers of a piano player striking the keys.(69) The nerve ends move in and out constantly with the speed and frequency communicated to them by the pressure of the ceaselessly repeated blows of luminous fluid which surges forward in high-velocity waves. The tiny nerves (or the fluid they contain), which are endowed with elasticity or contractility (or any specific mobility suitable to them), produce tremors and oscillations in varying rhythmic, harmonious movements. This variation in the tremors and oscillations in each nerve would produce a specific colour or tint in the soul, according to the hidden laws governing such a marvellous occurrence. The speed, number and size of the oscillations is, however, something that we do not wish to consider at the moment.

Whatever may be thought of this hypothesis, the irrefutable truth is that the light falling on the retina does not posit colours there - as we normally think it does - but only excites movements in the same way as the air, pressing wavelike upon the acoustic nerve, does not put sound in the ear, but simply sets the nerves in motion. When the nerves move in accordance with specific stimuli, then colour and sound sensations corresponding to that movement are felt in the soul. The same may be said of other species of sensation. We have to conclude, therefore, that the sensiferous phenomenon which takes place in our body and precedes the sensation is reduced to movements of a certain speed and frequency. The feeling phenomenon, that is, the sensation, takes place in the soul as a result of these movements in certain parts of the body.

132. It is now easy to see 1. that the movements aroused in our nerves are not the sensations themselves; 2. that the sensations cannot be connected with a single movement of these nerves.

In fact, if the sensations were the movements themselves, or adhered to the movements, the movements would feel themselves. As a result anyone who sees, would see and feel a trembling or oscillating sensation, and sound would reveal the number of oscillations by which it was produced. But this is absurd. Moreover, as the movement begins and gradually increases, sensation should be present, however delicate the movement, and increase and spread in accordance with the increase of the movement. This does not happen. Sensation only appears when the movement attains a certain speed, and vanishes totally. Again, a distinct sound is not dependent on the force with which the air strikes the ear. For example, oscillations produced with a frequency of 64 per second give us the sound of the first octave. But whatever the force present in such a frequency, the tone of the sensation will remain the same, despite the change in the strength of the sound. The sound varies in degree, but the tone does not. Sound sensation, therefore, is not produced by any one of the oscillations taken on its own but by many of them together.

133. The oscillations are many. On which of them does the single sensation depend? If on none of them in particular, it must depend upon all of them together. If so, the sensation must be simple, despite the multiplicity of the oscillations that pass through space. This also proves that the sensation is outside space; it cannot be located at any point in the lines followed by the oscillatory movement. The sensation does not have its seat in any point, but is aroused along with or after the spread of the whole movement. It remains outside all the points through which the movement passes.

It would be easy to carry on with this explanation, and compare movement and sensation under many other aspects. Many other arguments could be brought forward from other kinds of sensation to show that sensation is quite separate from the extension, succession and multiplicity of movement. But what we have said so far is sufficient to make perfectly clear that the feeling principle can only be simple and one.

134. Finally, an equally clear proof of the simplicity of the feeling principle is given by its identity in many sensations.

The same principle that hears sounds, hears many sounds simultaneously, and together with sounds has many sensations of sight, smell, taste, touch, hunger, thirst, and so on. This is certainly the case in the more perfect animals in whom, moreover, many sensations join to produce a universal feeling, different for each animal, which pervades and dominates the entire nervous system of the animal. We shall, however, speak more of this elsewhere. Meanwhile, it is clear that only a simple principle can draw together so many sensations and make itself the subject of them all. If the subject of the sensations were divided into two corporeal particles, even of the smallest dimensions, or as mathematical points if you wish, each point could feel only its own sensation, and not that of the other point. Hence, the identity of the feeling principle in many sensations clearly proves its perfect immateriality and simplicity.(70)

Notes

(53) OT, 871.

(54) `Although a single membrane receives taste impressions, the membrane itself is divided in two by a line separating two perfectly similar segments' (Bichat, Ricerche fisiologiche etc., p. 1, art. 2, §1).

(55) Woolaston offered the ingenious hypothesis that each optic nerve is divided to form immediately half of both retinas. Consequently the images outside the optic axis would come half from one eye and half from the other. There are serious difficulties against this hypothesis, as physics shows, but what concerns us here is the undoubted fact that when the two eyes are fixed on something directly in front of them, one part of the impression they receive corresponds to the middle part of the image which is equal for both eyes, although the person receiving the impressions sees a single scene, not a twofold scene. In Woolaston's hypothesis, therefore, the spirit through its own simplicity would have to compose a single image from four, rather than two images. Hence the difficulty remains.

(56) `Amongst all the organs, the brain by which we receive so many impressions is especially noteworthy because of the regular shape of its parts. These are exactly equal on both sides as we can see in the thalami of the optical nerves, and so on. The dissimilar parts are themselves divided symmetrically by a median line of which there are obvious traces in the corpus callosum, the arch with three pillars, the annular protuberance, and so on' (Bichat, Ricerche fisiologiche, etc., p.1, art. 2, §1).

(57) Riflessioni sulla conoscenza dello spazio che noi possiamo cavar dall'udito, inserted in his Indagine fisica sui colori, Modena, 1801.

(58) According to G. B. Venturi's experiments, sound is heard in the direction of the acoustic axis. This proves that the two impressions on the ears cannot be turned into one in the brain. The sensation of the single sound is truly composed of two sensations each of which is capable of being heard separately, as Venturi himself observes. Cf. Riflessioni sulla conoscenza dello spazio che noi possiamo ricavare dall'udito, in his Indagine fisica sui colori, Modena, 1801.

(59) The position of the plane of visual sensation is an evident argument for the wisdom of the author of nature. This position is exactly what is needed in order that the eye may help us to distinguish distances according to the laws of perspective.

(60) Note, however, that it is not necessary for the two impressions to occupy the same parts of each retina in such a way that the same point of the impression falls upon the same point of the retina. It is sufficient for the planes of the impressions to form whatever angle is necessary for the sensations to be aroused at the same points of the optic plane.

(61) Another law may be stated thus: `The impression is equal, that is, strikes the same points of the optic plane, when the two optical axes go to the same point.' The reason behind this law is that the luminous points from which the coloured rays start (each point is the vertex of the angle made by the optical axes) have their own distribution which is maintained by the rays as they move to the eye. If their respective position is disturbed (if, for instance, the ray going to one eye passes through one or more media, while the ray going to the other eye passes freely), the images are multiplied. The same happens if the rays are broken up by means of mirrors located at various angles, as in an kaleidoscope.

(62) According to the observations of Buffon and others, what is called a false ear arises principally from the inequality of the two acoustic organs. The two sound impressions are not perfectly equal and give rise to slightly differing sensations in the soul. The same phenomenon has been observed regarding other sensations. `If a cold inflames one nostril,' says Bichat, `and both nostrils remain open, the sense of smell is confused. It becomes distinct if the affected nostril is closed. A polyp on one side of the pituary gland weakens the efficiency of the nostril so that harmony between the two organs is defective and gives rise, as in the previous case, to confusion in the perception of smells.

`We can say the same about taste. Here too it often happens that we notice one side only of the tongue affected by paralysis or spasms. It would seem, therefore, that a middle line sometimes separates one feeling part from another which still retains its own sensitivity. If this takes place when infection has reached a high level, there is nothing to prevent us believing that it happens at a lower level. In a word, one side maintains perfectly its power of perceiving tastes, while the other has a lesser sense of taste. In this case, taste becomes irregular and confused as a result of two unequal sensations which, although induced by the same object, cannot induce a precise, exact perception. As a result, certain bodies, whose taste is doubtful, are the cause of pleasing sensations in some people, and of painful sensations in many others' (Ricerche fisiologiche, etc., p. 1, art. 2, §1).

(63) A good example of the way in which the soul joins two images into one is provided by the illusion produced by Wheatstone's stereoscope. Wheatstone observed that projecting a cube on the retina of each eye, when the cube is very near the eyes, produces impressions so different that it is almost impossible to recognise them as images of the same object. Nevertheless the result is a single cube because the two images are made one by the soul. If these two images are seen in appropriate positions, the illusion we have mentioned takes place: that is, we see a single solid cube produced in this case by the stereoscope. The explanation must lie in the nature of the optic plane, the immediate term of the feeling soul.

(64) If we restrict our examination of the eye to what we know about the direction of the optical axes, we can prove the existence of the law according to which the phenomenon occurs, but we cannot explain it.

(65) It will be objected that if the sensations of the two visual orbits adhere to the two optic nerves as other sensations do, the soul would be able to see them according to the distance between the optic nerves in which they are aroused. This is a mistake. We must remember that we are talking about visual not tactile sensations. The eye feels only something making an impression on the retina; it cannot therefore feel whatever exists between one optic nerve and the other. The distance between the optic nerves cannot be part of the visual orbits, nor separate them; that distance is not seen, and has no part in what is presented to the soul by the visual sensations.

(66) Protagoras was one of the classical authorities who saw the truth clearly, although he misused it. His arguments are set out by Plato (Theaetetus) in the words of Socrates who maintains that colour is found neither in bodies nor in the eye, but originates through movement, stimulated by bodies in the eye of the spectator. `As we have seen, what we perceive has a certain correlationship with the senses. The eye, which is constantly moving, generates a white colour in accord with what is like it. And so a stick or stone, or anything that appears white - or heavy or hot - to us, is nothing per se. Rather, all things and every change comes about as one succeeds another through movement. It is impossible to think that what acts or what suffers passively is of itself something unique or fixed, etc.' (cf. OT, 878-905).

(67) It is true that I also have said that the retina is designed or painted by light (Cf. OT, 907-922, where such expressions are used frequently), but I employ these common ways of speaking only when there is no fear that the reader will be misled. I do it for brevity. I will say, for example: `Let us imagine the white or yellow patch in the pupil', meaning that a white or yellow sensation corresponds in the soul with this impression. It would take too long to repeat everything constantly. Finally, I sometimes speak of the image in the retina, or of the colours scattered over and pitted upon the retina. Here I am not indicating the impression seen in the retina by others, but the visual sensation that, according to my opinion, is truly returned to the pupil immediately by the soul which, however, does not advert to its location as a result of the sensation until it can refer the sensation to external bodies through the use of touch.

(68) The retina is effectively a cellular net in the meshes of which are distributed the substance of the optic nerve. Darwin submitted the retina of a bull to the action of liquid potash and found that the nerve-part dissolved, leaving only the cellular net.

(69) There is, however, a difference to be noted between the tiny heads of the optic nerves and the keyboard of the piano. When a piano-key is struck, only one note is heard. But every tiny optic nerve would give any visual colour and tint sensations if it were activated and moved in the appropriate manner. This seems far more probable than supposing that there are different kinds of fibres in the eye for different colours if we consider:

1. The phenomenon of imaginary colours. If we look for a time at something white, and then place ourselves in darkness, the white sensation does not disappear immediately from the eye. It first changes into yellow, then red, then indigo, then blue, then green, until it finally fades away. This transformation of a white sensation into various successive colours offered G. B. Venturi an exemplary argument for concluding that different colours do not spring from different orders of fibres, but depend upon different movements of the same fibre (Indagine fisica sui colori, Modena, 1801, c. 5).

2. The analogy between imaginary colours and imaginary sounds. As a sound dies away in the ear, it changes into a sound harmonising with its predecessor. Does the ear also have different fibres on which sounds depend? Such fibres would presuppose different stimuli, but until now no way has been found of separating the soniferous liquid into seven species, as has been done with light, despite Mairan's ingenious hypothesis to this effect (in his Teoria del suono, 1737). Perhaps we could more reasonably say that the speed of the oscillations decreases according to a certain law, and that this is the cause of the imaginary sounds and colours? 3. The discovery that every colour becomes white when it is highly concentrated. This fact should perhaps be ascertained more definitely by new and more varied experiments.

(70) A fifth demonstration of the simplicity of the feeling principle can be drawn from properties of tactile and visual sensations, which extend only on the surface, as we shall see (cf. 177). But all the phenomena of animal life constitute proofs of the same truth.