CHAPTER 5

Criterion for the existence of bodies

749. After our analysis of the fundamental feeling and acquired sensations (in their subjective part), we can offer a more perfect definition of bodies.
First, however, the most famous definitions given by modern thinkers:
I. Berkeley and Condillac defined body as a complex of sensations. But as we have seen, sensation can be an effect only of a body's action on the spirit. The definition, therefore, lacks an agent, that is, the substance of a body, and retains only the accidental effect. But the substance of a body is body. The definition therefore excluded body and contained idealism, that is, the negation of bodies.

750. II. Descartes and Malebranche posited the essence of body in extension. The concept of extension however does not present any activity or force. On the contrary, it is the term of an action. Observation informs us that the first thing we experience of bodies is the feeling they produce in us by a certain action. Analysing this feeling, we find it refers to certain points in extension and spreads through and terminates in an extended element. Initially, therefore, we find extension to be like a mode of the feeling produced in us by bodies. It is true that if we analyse this mode of feeling (an effect of the action of bodies), as we will do in a moment, we also find that it must indeed be real in the cause that has produced it. Bodies must therefore be extended. This discovery is however a secondary discovery. According to Descartes the essence of a thing is that which we first conceive in the thing thought. But extension could not be thought unless we first thought of an action which, when done on us, reveals extension.

751. III. Leibniz saw that the essence of body had to be posited in some force. However, his argument did not begin from observation, which is the only starting point for a well-founded argument. Instead of being satisfied with the idea of a force acting on us and making us passive at the moment of corporeal sensation (a fact of consciousness), he imagined that body had to be a force acting not on us but only on itself through an internal energy, like all his other monads; body was a force acting in harmony with, but not on us. In this way, he removed the sole means for knowing the force, which is known only through observation of what is happening in us. The hypothesis that we ourselves form and develop within us some knowledge of the force is a gratuitous phantasy totally unsupported by observation, analogy or true intrinsic arguments. If bodies are to be conceived by imagination, not by observation, the forces we call bodies could indeed be fashioned in any way we like; we could suppose them to be simple(140) and endowed with perception. In this case, they would not be substances causing feeling but substances that feel. Leibniz's idea of body is therefore completely different from the idea I am presenting.

752. I start from observation because the description I give is intended to depend on this. Whether bodies have something that is outside our experience or our intellective conception of them is irrelevant.
Observation confirms that we are passive in sensations, that is, we experience an action of which we are not the authors. Consciousness of such an action is consciousness of a certain energy acting on us; and knowledge of such energy is knowledge of an ens, a substance. Hence the first but still imperfect definition we gave of body: 'a substance acting on us in a certain mode.'

To perfect this definition, we had to discover the meaning of 'in a certain mode', which we then inserted into the definition. We went on to analyse sensation because sensation or corporeal feeling is the action of this kind of substance.
The analysis showed a constant, uniform feeling(141) and an action partially modifying this fundamental feeling, that is, two actions, two energies, two substances, two bodies. Our own body produces the fundamental feeling and an external body modifies our body; we experience a body that is co-sentient as well as felt, and a body that is only felt.
The fundamental feeling, the action of our body, is not only a pleasant feeling, but is a pleasure with its own mode and limitation called extension, which does not derive from the simple notion of pleasure.
All acquired sensations are a species of touch.

Touch is both a subjective and an extrasubjective sensation because in it two things are felt: the sentient organ (the subjective part) and the external agent touching us and producing a sensation of touch (the extrasubjective part).
The subjective part is a modification of the fundamental feeling and makes us feel in a new, more intense way whatever part of our body is affected, while locating it at the same points as the fundamental feeling.

Furthermore, there are four classes or species of sensation particular to four organs of our body. They have four kinds of phenomena attached to them, colours, sounds, tastes and smells.
This analysis of the action of corporeal substance on us indicates that the essence of bodies must consist in: 1. pleasure and pain; and 2. extension, in which pleasure and pain are experienced. These are the two common, variable elements of the action. We can therefore improve our definition by saying: 'Body is a substance producing an action on us felt by us as pleasure or pain and having a constant mode called extension.'
To which we may now add: 'It can be accompanied by four kinds of phenomena called colour, sound, smell and taste'; but we should note that this addition does not mean these phenomena must be present. They are only an aptitude of a body to arouse them, given the necessary conditions.

753. Thus, such a substance, if firmly joined to us in the bond we call life (I am not investigating this here, whatever it may be), is a subjective body, exercising in our spirit a constant, uniform action called fundamental feeling. If this bond is absent, the substance is a foreign body, able to produce only partial, transient sensations.

754. With the establishment of the definition of body (cf. 752), we have also established the criterion for judging about its existence: 'I can say I am certain of the existence of a body when I am certain of the existence of that which, forming its essence, is expressed in its definition.'

755. In the first perception of our body we experience the feeling of life as pleasure or as the pleasant, individual union of a body with us.
This feeling, endowed with extension as one of its modes, is located at different points of space.(142) Thus by means of extension we perceive a body.

756. The existence of external bodies is proved in the same way. We perceive the two elements found in the definition of body.
The primal extension in which we locate our feeling undergoes some modifications from a cause different from us. In this modification we find
1. a partial, adventitious sensation of pain or pleasure which
2. is diffused in an extension more limited than, but not exceeding, the first extension.

Sometimes the phenomena of the four organs, eyes, nose, ears and palate are also present, if the organs are stimulated.
These conditions once more confirm our perceptions of an external body.

757. A sensation of pleasure or pain by itself does not indicate the presence of a body. It tells us that an action is being done in us and that the action must have a cause different from us, but of itself it would never tell us that this cause is a body, because the essential element of extension would be missing. The sensation must be capable of making us perceive an extension if we are to have a corporeal sensation. Extension determines our sensation, making it a corporeal or material sensation.
And vice-versa: extension by itself is not body, since the first essential element of body is the energy for producing a feeling in us.
To avoid making a mistake about the existence of a body, we must verify for ourselves the two following conditions or elements that form its essence: 1. a feeling (our passivity, external action); 2. an extension to which the feeling is referred (mode of the feeling).

758. There is an action done in us that constitutes the fundamental feeling; joined to this feeling is the mode of extension.
Thus a body exists permanently united to us. Its existence is no longer subject to doubt because we cannot be deceived as to whether we are alive or dead; the two elements constituting our body in this case are two facts of consciousness.
In adventitious sensations we distinguish:

1. a modification of the fundamental feeling, that is, a new, more intense sensation of some part of our body; and
2. a perception of an agent outside the extension in which our fundamental feeling is diffused.

The modification is the second subjective way of perceiving our body; the perception is the extrasubjective perception of external bodies.
The existence of our body therefore is always founded on the evidence of the fundamental feeling.

759. The certainty of the existence of external bodies is also founded on the fundamental feeling, because their action on us is indivisibly joined to the modifications of the feeling, while their extension is measured by the extension first occupied by the fundamental feeling.

760. Our body, therefore, perceived in the first mode, becomes a criterion for the existence of all other bodies.
The other modes of perceiving a body must be reduced to this first mode, that is, perception by the fundamental feeling. Thus, the second subjective mode is reduced to the first because it is a modification of the fundamental feeling, and the third extrasubjective mode (for external bodies) is reduced to the first because the extrasubjective extension becomes known through a comparison with the subjective extension.

761. We cannot err about the existence of our own body perceived in the first mode, that is, with the fundamental feeling (cf. 755-759).
We can be misled about the existence of some part of our body when it is perceived by acquired sensations. A perception of this kind includes the other two modes, the subjective and the extrasubjective (cf. 760).

762. For the moment I am not concerned with possible error in the third mode, that is, in perceiving our body as an external agent rather than as a subject. This error, common to the perception of all external bodies, will be dealt with later.
For the moment I want to examine possible error concerning the existence of some part of our body perceived in the second subjective mode. For example, an amputee acutely feels the pain of a lost hand or foot, not in the stump but in the limb that still seems to be there. In this case the person locates the pain deceptively and wrongly at the extension.
The error can be discovered by applying the criterion.(143)
The amputated limb is not felt by the fundamental feeling but by the adventitious sensation of the pain. To know whether such a sensation is misleading, it must be reduced, as we have said, to the fundamental feeling as its criterion and proof.
This is done when we verify that the acquired sensation is a modification of the fundamental feeling.
In the case of the amputee, the sensation of pain in the arm or leg is certainly a modification of the fundamental feeling but this fact does not prove the existence of our body (cf. 757); the extension felt by the sensation must be capable of being reduced to the extension of the fundamental feeling.

Now we have noted that there are two characteristics of the fundamental feeling: 1. its constant, uniform existence; and 2. its aptitude to be modified. By applying this second characteristic, let us see if the extension of the amputated limb is felt in reality.
If the limb we perceive is the same limb felt by the fundamental feeling, it must be subject to modifications, because the fundamental extension (our hand perceived by the fundamental feeling) is essentially modifiable. If then the hand exists, it can be touched, seen, moved, etc. because these are modifications of the fundamental extension. But this cannot happen with the amputated hand - it is felt, but not by the fundamental feeling. It is a misleading phenomenon since it cannot be reduced to the fundamental extension nor shown to be a modification of it. Indeed when I feel my hand through the pain I experience in it, the mode of this sensation, that is, extension, must be identical with the extension of the fundamental feeling; the only other possible difference is that the sensation in the fundamental feeling endures and is less vivid, while the acquired or adventitious sensation is more intense, partial and transient.

763. The idealists' argument, drawn from what we see in dreams, is clearly without foundation. They ask: could life not be one long dream?
They do not observe that the images in dreams may mislead us about the existence of external bodies but not about the existence of our own body; in fact, they contest this.
The illusions of dreams are the result of the body's being stimulated in a certain way and would therefore be impossible if we did not have a body. They do not cast doubt on the existence of bodies in general; on the contrary, they prove and confirm their existence. Later we will see how to distinguish between what is false and true in external phenomena.

(140) Later I shall directly refute the simple points of Leibniz and Boscovich.

(141) The fundamental feeling is modified as the human body grows.

(142) To say the feeling is located at different points of space is the simple way of indicating space perceived in a shaped way. But calling extension simply a mode of feeling keeps us within the subjective sensation of extension.

(143) The cause of this error lies not in the sensation but in an habitual judgment. When we still had our hand or foot, the pain we felt was referred to them by a necessity of nature because that was where we felt the pain. This necessity then became a habit which remained even when the necessity had disappeared. And because we now feel a pain that is no different from the previous pain in our hand or foot, we think it is the same and assign it to the same place without adverting to the real place.