Objection 1. It seems that falsity is not in the senses. For Augustine says (De Vera Relig. 33): "If all the bodily senses report as they are affected, I do not know what more we can require from them." Thus it seems that we are not deceived by the senses; and therefore that falsity is not in them.
Objection 2. Further, the Philosopher says (Metaph. iv, 24) that falsity is not proper to the senses, but to the imagination.
Objection 3. Further, in non-complex things there is neither true nor false, but in complex things only. But affirmation and negation do not belong to the senses. Therefore in the senses there is no falsity.
On the contrary, Augustine says (Soliloq. ii, 6), "It appears that the senses entrap us into error by their deceptive similitudes."
I answer that, Falsity is not to be sought in the senses except as truth is in them. Now truth is not in them in such a way as that the senses know truth, but in so far as they apprehend sensible things truly, as said above (Question 16, Article 2), and this takes place through the senses apprehending things as they are, and hence it happens that falsity exists in the senses through their apprehending or judging things to be otherwise than they really are.
The knowledge of things by the senses is in proportion to the existence of their likeness in the senses; and the likeness of a thing can exist in the senses in three ways. In the first way, primarily and of its own nature, as in sight there is the likeness of colors, and of other sensible objects proper to it.
Secondly, of its own nature, though not primarily; as in sight there is the likeness of shape, size, and of other sensible objects common to more than one sense.
Thirdly, neither primarily nor of its own nature, but accidentally, as in sight, there is the likeness of a man, not as man, but in so far as it is accidental to the colored object to be a man.
Sense, then, has no false knowledge about its proper objects, except accidentally and rarely, and then, because of the unsound organ it does not receive the sensible form rightly; just as other passive subjects because of their indisposition receive defectively the impressions of the agent. Hence, for instance, it happens that on account of an unhealthy tongue sweet seems bitter to a sick person. But as to common objects of sense, and accidental objects, even a rightly disposed sense may have a false judgment, because it is referred to them not directly, but accidentally, or as a consequence of being directed to other things.
Reply to Objection 1. The affection of sense is its sensation itself. Hence, from the fact that sense reports as it is affected, it follows that we are not deceived in the judgment by which we judge that we experience sensation. Since, however, sense is sometimes affected erroneously of that object, it follows that it sometimes reports erroneously of that object; and thus we are deceived by sense about the object, but not about the fact of sensation.
Reply to Objection 2. Falsity is said not to be proper to sense, since sense is not deceived as to its proper object. Hence in another translation it is said more plainly, "Sense, about its proper object, is never false." Falsity is attributed to the imagination, as it represents the likeness of something even in its absence. Hence, when anyone perceives the likeness of a thing as if it were the thing itself, falsity results from such an apprehension; and for this reason the Philosopher says (Metaph. v, 34) that shadows, pictures, and dreams are said to be false inasmuch as they convey the likeness of things that are not present in substance.
Reply to Objection 3. This argument proves that the false is not in the sense, as in that which knows the true and the false.
Source: Summa Theologica (New Advent)