A brief consideration of Aquinas’s account of perceptual error regarding sense objects is in order. A discussion of non-veridical perception, albeit brief, is needed here because Aquinas considers perceptual error in regard to both the mental act itself and the object of the mental act. The distinctions he develops regarding non-veridical sensation and perception depend upon the categories of objects of sensation and perception. Aquinas admits non-veridical awareness of objects regarding objects of sensation or perception in only two of the three categories of objects. On the other hand, he admits the possibility of a non-veridical awareness into each category of mental act. Accordingly, if ‘to be aware of’ is taken to be a ‘success term’, Aquinas argues that in all three categories of mental acts themselves, the function of the mental act may not be successful:
- (a) Regarding the objects, there can be a non-veridical awareness of an object of sense knowledge in only two categories of these objects: the common sensible and the incidental object of sense.
- (b) Regarding the mental acts themselves, however, there can be non-veridical, unsuccessful awareness in all three categories of awareness: the awareness of a proper sensible, the awareness of a common sensible, and the awareness of an incidental object of sense.
In the Summa Theologiae, Aquinas considers non-veridical sensation: ‘Expect to find error in the senses to no greater extent than truth. Truth is in the senses not because they can know what truth is, but because they have a true perception of sensible objects. Similarly error enters when the senses apprehend and judge things to be other than they are’ (Summa Theologiae, I q. 17 a. 2). He explains what is necessary for a ‘true perception of a sensible object’:
Sense knows things from being impressed with their likeness. Now this likeness can be taken at three stages:
First, immediately and directly [primo et per se], as when the likeness of colour is in the sight. [ . . . ]
Secondly, directly, but not immediately [per se sed non primo], as when the likeness of bodily shape or size is in the sight. [ . . . ]
Thirdly, neither immediately nor directly but indirectly [nec primo nec per se, sed per accidens], as when the likeness of a human person is in sight. She is not there because she is a human person, but because she is a coloured object [sed in quantum huic colorata accidit esse hominem]. (Summa Theologiae, Ia q. 17 a. 2; emphasis added)
Aquinas concludes this analysis by considering error regarding the objects of sensation:
Now to apply this distinction. We say that the senses are not deceived regarding their proper sense objects, except by interference, in abnormal cases, and when the sense organ is impaired.
Regarding the common sensibles and incidental objects of sense, however, there can be erroneous sensations even in a healthy sense. For the sense is not immediately related to them, but only incidentally, namely, in consequence of their being involved in its primary function. (Summa Theologiae, la q. 17 a. 2)
In Aquinas’s view, non-veridical awareness with sense objects can occur with the common sensibles and the incidental object of sense. But it must be noted that this discussion concerns only error regarding objects of sensation; it does not concern the possibility of error regarding the mental act itself. This is why in the above passage Aquinas mentions that the proper sensibles are always perceived veridically except in the cases when there is ‘interference and [ . . . ] when the sense organ is impaired’ When the act of perception is analysed, this discussion will be further elucidated. It is not too early, however, to mention that ‘interference’ refers to the ‘medium’ needed for each act of sensation, and ‘impaired sense organ’ refers to the ‘adequately disposed sense organ’. In the Commentary as in the Summa Theologiae, Aquinas regards non-veridical sensation regarding objects as being possible only in two categories of sensible objects and not with the proper sensibles:
First, about its proper object, sense is always true or only slightly false. Natural powers are not unable to perform their proper activities, except in the minority of cases on account of illness or injury. So the senses are not deficient in judging their proper objects, except sometimes because of impaired organs, as when the ill-disposed tongue of a feverish patient makes what is sweet taste bitter. (Commentary on the Soul, no. 661)
Aquinas further elaborates his position:
But the senses can be deceived both about objects only incidentally sensible and about objects common to several senses. Thus sight would prove fallible were one to attempt to judge what a coloured thing was or where it was. And hearing likewise if one tried to determine by hearing alone what was causing a sound. Such then are the special or proper objects of each sense. (no. 385)
Aquinas next treats the common sensibles: ‘A sense also has to deal with the common sensibles. [ . . . ] Size, for instance, and motion are common sensibles of bodies. This judgement might vary according to the differences of distance and misjudgement is easy’ (Commentary on the Soul, no. 663). Lastly, Aquinas offers his account of the incidental object of sense: ‘A sense has to deal with what is incidentally sensible, and here it may be deceived. In seeing white the sense may be deceived as to whether it be snow or flour or something of the sort. Mistakes are especially easy with regard to strange or distant objects’ (no. 662). These texts express clearly Aquinas’s position that non-veridical sensation in regard to objects is possible structurally only within two categories of objects; only the object of sensation is being considered and not the act itself. Even though Aquinas affirms that the perceiver is never wrong—i.e. never non-veridically perceiving—regarding the proper sensibles, he immediately qualifies this statement insofar as the perceiver is what a contemporary philosopher might call a ‘normal’ perceiver under ‘normal perceptual conditions’.
Two contemporary philosophers have commented on this set of issues. Kenny writes: ‘The traditional impossibility of mistaken sense-perception was restricted to such sense-objects as sounds and colours; it did not extend to the natures of the events or objects which were the causes or bearers of the sounds or colours.’19 Pasnau suggests that Aquinas handles the cases of ‘sensory infallibility [. . .] by carefully qualifying the scope of the infallibility’.20 This furthers the analysis noted above in the discussion indicating the necessary conditions for ‘sensory infallibility’ regarding the proper sensibles: (a) the sense organ is in its proper functioning condition; and (b) there exists no impediment to the possibility of the organ’s reception of the proper sensible. Pasnau suggests the following about the justification for this set of claims by Aquinas: ‘Jointly, these two restrictive conditions block all the obvious counterexamples to the alleged infallibility of the senses. [And this] amounts to the claim that in ideal circumstances, functioning as they should, the senses do not make mistakes about sensible qualities,’21 An evolutionary and ecological psychologist like Gibson could argue that in order to survive, the senses have been developed ‘by nature’ to be reliable instruments for the awareness of proper sensibles; this enables the animal or human to make its way successfully around the environment. This is a limited but probably correct account of what Aquinas holds about the teleological structure of the sense faculties. The structural similarity with Gibson’s account of perception is striking.
-  This is a hint that Aquinas argues for a structured mental act in perception rather than opting for the‘diaphanous arrow of consciousness’ so prevalent in the early analytic work of Russell and Moore. Plato alsoaccepts a diaphanous mental act in his theory of recollection that is structurally similar to Russell’s ‘principleof acquaintance’.
-  In the Summa Theologiae, Aquinas spells out how error might occur with a proper sensible: ‘For senseis not deceived in its proper object, as sight in regard to colour, unless accidentally through some hindranceoccurring to the sense organ—for example, the taste of a fever-stricken person judges a sweet thing to bebitter, through his tongue being vitiated by ill humours. Sense, however, may be deceived as regards common sensible objects, as size or figure; when, for example, it judges the sun to be only a foot in diameter,whereas in reality it exceeds the earth in size. Much more is sense deceived concerning incidental sensible