Appendix 38.

(947) [Erroneous judgments about sensations]

The idealists in particular have abused this impropriety. As we can see in Hume, they drew one of their arguments from the changing size of bodies as distance changes.
Aristotle is a good example of the antiquity of this deception which attributes to all the senses in general what pertains only to sight. He says that size and movement are common sensible qualities, and that our sense is deceived much more by these qualities than by the sensible qualities proper to each sense. His reason is that sizes and movements change as distances change. That which is proper only to the eye, which Aristotle calls the greatest sense (De Anima, bk 3), is attributed to the senses in general. But he should really have said that applying colours to bodies is a far more frequent error, because colours in fact are only sensations in the optic nerve. Evidently Aristotle did not notice this; he too accepted the general error.

I will use this opportunity to add another observation about the defects, they seem to me, in Aristotle's analysis of sensations. Apparently he was not always aware of the habitual judgments we continually mix with sensations. Like people in general, he confused them with sensations. For example, he says, 'Only on very few occasions does sense mistake its proper objects.' The explanation commonly given of 'very few occasions' (the context and Aristotle's style of language do not allow any doubt) is that sense mistakes its proper objects only on the very rare occasions when it is infirm. But note, infirm sense in itself does not err; error is caused by the judgment we add to sense. Thus sense, even when infirm, does not mistake its proper objects. The error lies in our judgment, which is the source of every error. Aristotle's teaching must give place to that of St. Augustine who says: si omnes corporis sensus ita nuntiant ut afficiuntur, quid ab eis amplius exigere debeamus, ignoro [If all the senses of the body tell us in this way that they have been affected, what more can we possibly require from them?] (De Vera Religione, c. 33).