Appendix 33.

(897) [Advertence and senses]

The following observations offer further confirmation that many things attributed to the different grades of perfection of our senses should in fact be attributed to levels of perfection of our advertence or attention to sensations.
Observation shows that the hand is not the most sensitive part of our body; other parts contain more nerves and are more sensitive. In fact we can say that the skin is more sensitive in all other parts of our body than in the hand where nature has wisely reduced sensitivity so that we can use the hand freely without being frequently troubled by pain. Continual use hardens our hand still more. Increase in the hand's sensitivity, therefore, is not brought about by its use except for greater effectivity and alertness of the nerves in the parts that are used more; and I have no doubt that this comes into play here. This, however, only proves the need for a greater attention or at least sensitive effectivity. My concern, on the other hand, is to know which part of our body can more easily make us perceive and distinguish the tiniest particles, the little inequalities of rough, uneven bodies and all their tactile differences. The answer must be the hand.

This ability does not come from greater, natural sensitivity of touch in the hand but from our habit of using the hand for that purpose and from our habit of adverting to minute differences in the hand's sensations. Normally, we do not learn to advert to them in other parts of our body. But extraordinary things are done by people who have lost their hands. Long education has taught them to be attentive to the sensations in their feet, accurately noting and differentiating every sensation. Such evidence does not lead me to think that sensitivity in their feet has increased. Rather, they have learnt to direct their attention to the sensations in their feet and note what takes place. Other people pay little or no attention at all to these sensations.

A good doctor with long experience notices the least change of pulse in a sick person; others do not. His sense of touch has not been refined specifically for taking pulses; any other person could have taken the pulse and learnt nothing. If the constant taking of pulses actually refines the doctor's touch, why does contact at the place of the vein rather than elsewhere render the nerves of the doctor's hand more sensitive? Why is the doctor's touch so sensitive to pulses but dull and coarse to the delicate engravings of an object worked in gold. If feeling the differences of pulse depended on the physical sensitivity of the skin and not on the acquired ability to advert to what is felt, the same sensitive touch could be used for everything; those born blind would not have to learn to determine the pulse by practice since they would already have a very sensitive touch.

All this is explained by advertence to our sensations, which is being continually improved and increases much more than the senses themselves. Our senses might improve a little even physically by use but certainly not enough to explain the great difference between senses that have been used meaningfully and senses that have not been so used. The physical refinement of a sense, which depends on the texture of the organ, is given by nature and cannot be markedly changed. The sense of sight, it seems, can be improved and sharpened by practice, but we must remember that what this sense tells us about distant bodies is due to habitual judgments (as we shall show later); it is our ability to make these judgments that is perfected. And what sight tells us about surfaces is to a large extent due to the practice of observation. The jeweller's sharp eye, the doctor's capacity to tell at a glance how a person feels, other people's insight into character, all depend upon sharpened observation. The very subtle differences painters see in colours and pictures obviously involves skill in discerning the variations which others also see but do not advert to.

The same is true about music and the practised ear, which seems to hear much more than the ears of others in an orchestral piece. In both cases the same sounds are perceived but with different mental attention. Another example is the acute sense of smell of tribesmen who, it is said, could identify the tracks of the Spaniards by smelling the ground. But we should be much more amazed by their constant practice in sensing minute sensations of smell and their differences.

Again, the taste buds of the palate undergo little change from frequent contact with different foods but what amazing sensitivity gourmets acquire, compared with other people, in judging different tastes! Perhaps Juvenal's glutton had dulled his palate by continual use of spices and dainty foods, but nevertheless, by applying the greatest attention to food, had so developed his sense of taste for oysters that one gulp could tell him from which sea they had come.

The benefit the reader can draw from all these observations is to be convinced of the great difference between sensation and advertence, and to be persuaded that we feel an infinity of things without being in the least aware of them.