Home Economics Aquinas’s theory of perception: an analytic reconstruction
The Common Sensible and the Incidental Object of Sense
A second problem emerges when the ‘common sensibles’ are contrasted with what both Aristotle and Aquinas refer to as the ‘incidental object of sense’. In the previous analysis, both the proper and the common sensibles were classified as Sensation-Ia objects, and the incidental object of sense was classified as a Sensation-Ib object. This distinction separates the directly sensed objects and the indirectly perceivable objects. The problem arises when one wonders why the common sensible is a Sensation-Ia objects, as when it judges that vinegar is honey by reason of the colour being the same. The reason of this is evident; for every sense faculty, as such, is “per se” directed to its proper object; and things of this kind are always the same. Hence, as long as the faculty exists, its judgment concerning its own proper object does not fail’ (Summa Theologiae, Ia q. 85 a. 6).
rather than a Sensation-Ib object. In other words, both Aristotle and Aquinas claim that each sense faculty is directly ordered to a proper sensible. How, then, is it possible that the common sensible is also a Sensation-Ia object? To explicate this issue, both the incidental object of sense and the common sensible itself must be considered in detail.
In On the Soul, Aristotle offers a pithy remark concerning the incidental object of sense; he spends little time offering an analysis of this sense object. Aquinas develops this analysis in more subtle and detailed ways. ‘We speak of an incidental object of sense where, e.g. the white object, which we see, is the son of Diares here because “being the son of Diares” is incidental to the directly visible white patch; we speak of the son of Diares as being (incidentally) perceived or seen by us. Because this is only incidentally an object of sense, it in no way as such affects the senses’ (On the Soul, 418a20). The incidental object of sense, Aristotle suggests, appears to be the particular or individual thing existing in the external world—a primary substance. There is an awareness of an individual as an individual and not as a colour patch or a bundle of sensations. In De Veritate, Aquinas writes: ‘For a human person exemplifying a colour and a stone exemplifying a colour are perceived by the same sensitive faculty, since it is incidental to the sensible object in so far as it is a sensible object [i.e. a proper sensible], to be a human or a stone’ (De Veritate II, 1, 15). Aquinas comments on this Aristotelian passage:
We might, he says, call Diares or Socrates incidentally a sense object because each happens to be white: what is sensed incidentally [sentiturper accidens] which happens to belong to what is sensed absolutely [sentitur per se]. It is accidental [i.e. incidental] to the white object, which is sensed absolutely, that it should be Diares; hence Diares is a sense object incidentally. He does not, as such, act upon the sense at all. (Commentary on the Soul, no. 387)
The important note emerging from these texts is that the incidental object of sense is not directly apprehended per se by the external senses. The incidental object of sense is not a Sensation-Ia but rather a Sensation-Ib object. But it must also be noted that the lack of direct apprehension of the incidental object of sense refers explicitly to the external senses. This will be quite important in a later discussion when Aquinas argues that the vis cogitativa is the faculty by means of which a perceiver is directly aware of an individual concretum. Aquinas is not proposing representationalism; he is only denying that the external senses alone are the means by which a perceiver is directly aware of an individual as a concretum and not as a mere bundle of sensations. This is a further indication that the mental acts of the internal sensorium are connected structurally with Aquinas’s account of perception. He appears concerned about the paucity of the Aristotelian account of the incidental object of sense and how this object is related to the common sensibles. In the following passage, Aquinas attempts to explain how the proper sensibles determine the external sense faculties. This explanation is important as a structural foundation upon which to clarify the difference between the incidental object of sense and the common sensible. It was indicated earlier that the proper sensibles and the common sensibles are placed in the category of Sensation-Ia objects. Structurally, the common sensible is distinct belongs to a different category from the incidental object of sense, which object is a Sensation-Ib object. Aquinas comments on this issue:
While it is true, however, that both common and proper sense-objects are all absolutely or by and of themselves perceptible for a sense, yet, strictly speaking, only the special or proper sense objects are directly perceived [proprie per se sensibilia], for the very essence and definition of each sense consists in its being naturally fitted to be affected by some such special object proper to itself. The nature of each faculty consists in its relation to its proper object. (Commentary on the Soul, no. 387)
This passage brings to light the problem under consideration. The distinction mentioned in the preceding passage suggesting that the proper sensibles are the only objects that are directly perceived creates a philosophical difficulty. Quite possibly, both the common sensibles and the incidental objects of sense are equally only ‘incidentally sensible’. Aquinas denies this possibility when he argues for the inclusion of the common sensibles into the category of Sensation-Ia objects.
We have seen that sensation is a ‘being acted upon’ and ‘altered’ in some way. Whatever, then, affects the faculty in, and so makes a difference to, its own proper reaction and modification has an intrinsic relation to that faculty and can be called a sense-object in itself or absolutely [Sensation Ia objects]. But whatever makes no difference to the immediate modification of the faculty we call an incidental object [Sensation Ib objects]. Hence, the Philosopher says explicitly that the senses are not affected at all by the incidental object of sense as such. (Commentary on the Soul, no. 393; emphasis added)
The Summa Theologiae texts noted above consider the different ways in which the three sensible objects affect the sense organs: (a) the proper sensible directly and immediately; (b) the common sensible directly., but not immediately; (c) the incidental object of sense neither directly nor immediately but incidentally. These three categories indicate the different uses of cause regarding sensation and perception. It should be noted that some commentators on Aristotle—and this is extended to Aquinas—argue that the common sensibles are the sense objects of the sensus communis (what is often translated as the ‘common sense’), and are perceived by the external senses only through the efficacy of the sensus communis.22 Furthermore, this is not Aquinas’s position, as he makes clear in his Commentary on the Soul. However, in his Summa de Homine (35.4), his Dominican mentor, Albertus Magnus, appears to have held this position on the common sensibles. 
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