Home Economics Aquinas’s theory of perception: an analytic reconstruction
Organ and Faculty
Aquinas distinguishes between the sense organ and the sense faculty. The sense organ is the bodily part which houses, as it were, the sensory machinery, which is a necessary condition for sensation to occur. With visual sensation, this would be the pupil of the eye. Aristotle often referred to this as the ‘eye jelly’. Within the pupil is the sense faculty, which in Aquinas’s philosophy of mind is the means by which the actual sensible object is known. The faculty is the sensible power that is capable of receiving a sensible form without matter, which is, of course, Aquinas’s basic ‘principle of inten- tionality’. He writes frequently that ‘the sense faculty is always the function of a bodily organ’ (Commentary on the Soul, no. 377), and notes explicitly this distinction between organ and faculty: ‘for the principle of vision is not the sense of sight only, but the eye consisting of pupil and the faculty of seeing. But it needs a body objectively, as the sight needs the wall on which is found the colour’ (On the Power of God, bk I, q. 3 a. 9 rp. 22). In his Commentary Aquinas discusses the difference between organ and faculty: what follows are primary texts in which Aquinas discusses this organ/faculty distinction. In his long discussion against the followers of Averroes, Aquinas wrote: ‘Sense is proportioned to its organ and is in a way akin to it in nature; hence with the immutation of the organ the operation of sense too is changed. It is thus then that the phrase, “is not mixed with body”, should be understood: intellect does not have an organ as sense does’ (On the Unity of the Intellect Against the Averroists,
ch. 1, no. 23). In the Compendium of Theology, Aquinas discussed organ and faculty in the following manner:
There are other powers whose operations do not transcend the limits of bodies and yet extend to the species of bodies, receiving them without their accompanying matter. This is the case with all the powers of the sensitive soul. For sense is capable of receiving species without matter, as the Philosopher says. But such [sense] faculties, although they are receptive of the forms of things in a sort of immaterial [intentional] way, do not receive them without a bodily organ. If procession takes place within these powers of the soul, that which proceeds will not be something corporeal, nor will it be distinct or joined to that faculty whence it proceeds in a corporeal way, but in a certain incorporeal and immaterial [intentional] fashion, although not entirely without the help of a bodily organ. Thus the representations of things imagined, which exist in the imagination not as a body in a body, but in a certain spiritual [intentional] way, proceed in animals. This is why imaginary vision is called spiritual by Augustine. (Compendium of Theology, pt I, ch. 52)
This passage indicates that Aquinas is not a reductive materialist. Moreover, this claim of ‘the sense being a certain ratio’ occurs often in Aquinas’s texts on the philosophy of mind. In medieval philosophy, ‘ratio’ indicates the structure or form of an object. Hence, the faculty or power, as form, is the intentional structure of the sense organ that enables it to receive the sensible object intentionally. This is analogous to Chisholm’s ‘funny characteristic’ discussed earlier. The organ is the ‘matter’ or the physical placeholder for the faculty. Yet the organ too, as a disposed power or potency, has a ‘ratio’ in terms of its physical apparatus. It is this use of ‘ratio’ that Aquinas appeals to in these texts. Aquinas, following Aristotle, then repeats his claim that an excessively strong sensible object can disrupt the sense organ:
Aristotle explains why excess in the object destroys the sense organ; for, if sensation is to take place there must pre-exist in the organ of sense ‘a certain ratio’ or, as we have termed it, proportion. But if the impact of the sense object is stronger than what the organ is naturally able to bear, the proportion is destroyed and the sense itself, which precisely consists, as has been said, in the formal proportion of the organ, is neutralized. It is just as though one were to twang cords too violently, destroying the tone and harmony of the instrument, which consists in a certain proportion. (Commentary on the Soul, no. 556)
Later in the Commentary, Aquinas reiterates this claim: ‘For a very strong sense object can stun the faculty of sense. One can be deafened by great sounds, blinded by strong colours, made powerless to smell anything by overpowering odours; and this because the organ in each case is injured’ (no. 688).
The role of this ‘proportion’ or ‘ratio’ is discussed within a long passage in the texts, albeit abbreviated here, with Aquinas expressing his disagreement with the Latin Averroists: ‘The sense is proportioned to its organ and in some way is assimilated to its nature. Therefore, the operation [the act of mental awareness] is changed even according to the change of the organ. And [. . .] the intellect does not have an organ as the sense does’ (On the Unity of the Intellect Against the Averroists, ch. 1, sec. 23). In the Compendium of Theology, one finds the following account of this issue, with emphasis on the intentionality of sense knowledge.
There are knowing powers whose acts of awareness do not transcend the limits of bodies, and yet they extend to the species of other bodies, receiving them without their accompanying matter. This is the case with all the powers of the sensitive soul. For sense is capable of receiving species without matter, as the Philosopher says. But such faculties, although they are receptive of the forms of things in a sort of immaterial [intentional] way, do not receive them without a bodily organ [. . .] and not entirely without the help of a bodily organ. (Compendium of Theology, pt I, ch. 52)
Both the external senses and the internal senses require both faculties and organs, which the following passage indicates:
Avicenna has reasonably shown that the faculties [i.e. of imagination and memory] are distinct. Since sensitive faculties are acts of physical organs, it is necessary that the reception of sensible forms, which pertains to the external senses, and their conservation, which belongs to the imagination [or phantasia] pertain to distinct faculties. As we note in physical things, (a) reception pertains to one principle, and (b) conservation to another. [Aquinas gives the example that humid things are quite receptive, but dry and hard things are less receptive.]
In a similar manner, it pertains to one principle to receive a form, to another to conserve the form received by the senses, and to still another to perceive some signification not apprehended by the senses. The vis aestimativa perceives the meaning [i.e. that not apprehended by the external senses] [. . .] and the vis memorativa retains this meaning. The vis memorativa functions by remembering a thing, not absolutely, but as it was apprehended in the past by the senses or the intellect. (Sententia Libri De Memoria et Reminiscentia, lesson 2, no. 321)
In the Summa Contra Gentiles, Aquinas discusses the need for sense organ and sense faculty:
Different senses are receptive of different sensibles, sight, for instance, of colours, hearing of sounds. Now this difference clearly arises from the different dispositions of the organs: for the organ of sight needs to be in potentiality to all colours, and the organ of hearing to all sounds. But if this reception took place without any corporeal organ, the same faculty would be receptive of all sensible objects. (Summa Contra Gentiles, bk II, ch. 82)
What is one to make of all of this? One needs to begin with the sensible object in the external world, for example, a red apple. The ‘red’ is an ‘active power’ in the primary substance, which primary substance would be an instance of this particular Jonathan apple. This ‘red’ is an accidental or incidental form of the apple. As a ‘proper sensible’, red functions as a ‘kind’ of agent which will interact with the eye; a common sensible will interact as a ‘mode’ of activity. The active power is causally efficacious and can act with the eye to produce an awareness of the colour of red.
The eye is both organ and faculty. It is a ‘passive power, which means it functions in the role of a ‘patient’ and the ‘colour’ is the agent or ‘active power’. The organ is a bodily capacity to be affected in a material way. But, Aquinas insists, against contemporary physicalists and the atomists of his day, sensation involves more than a bodily interaction. He appeals to a matter/form analogy, as he and Aristotle do so often, to explain this distinction between organ and faculty. The faculty is the ability to receive the sensible form without its matter. The faculty is the ‘form’ of the organ, and the organ itself serves as the ‘matter’ for sensation. This means that the faculty as form is a disposition to receive the form of the sensible, but in an intentional manner. This is where the ‘esse intentionale’ comes into play for sensation: ‘Every subject of passion receives the action of the agent according to its mode. Accordingly, if there is a thing that is naturally adapted to be altered by an active principle, with both a natural and a spiritual [i.e. intentional] alteration [. . .] natural alteration precedes spiritual [intentional] alteration, just as natural precedes intentional being’ (Summa Theologiae, Supp. to III, q. 82 a. 3 ad 2). In this text, it is unclear whether both the organ and the faculty are ‘ratios’ or just the organ itself; nonetheless it would appear that both have some kind of structure. Aquinas speaks about the ‘ratio’ of the organ, which can be impaired by too strong and intense a sensible—too bright a colour for the eye, too harsh a sound for the ear, too hot a sensation of heat for the touch, and so forth. Yet the sense faculty appears to be a kind of ratio too insofar as it is directed teleologically to one kind of proper sensible rather than another. Nonetheless, Aquinas notes in the Compendium that a change in the sense organ produces some indirect modification in the sense faculty. One wishes Aquinas were clearer on this causal relation. ‘All the powers of the sensitive part of our soul, whether they are apprehensive or appetitive, are the acts of certain bodily organs. If these undergo modification, the faculties themselves must, indirectly, undergo some change’ (Compendium of Theology, pt I, ch. 128). All of this is important in explicating Aquinas’s account of direct sensation. This is how Aquinas becomes a ‘direct realist’ and still maintains a form of a correspondence theory of truth.
In order to avoid the classic problems in sensing with representationalism, Aquinas argues for the ‘identity’ of the sensible faculty with the sensible object itself. Both the primary substance and the sense faculty or power exemplify the incidental quality; one exemplification is material (esse naturale) and the other exemplification is intentional (esse intentionale). Aquinas takes his cue from Aristotle, who suggested that when sensing, the faculty is like the saw sawing. Take the case of ‘seeing red’: the act of the faculty is the exemplification of the form of the proper sensible, red, impressed on the sense faculty. This is the same form, exemplified in the particular material object, which is a Jonathan apple. In seeing red, the faculty has an intentional existence of the form—i.e. the form is realized intentionally—which is the same form as the form in the apple. The act of seeing is the sensible object. In other words, seeing without colour is never seeing.
Here Aquinas, following Aristotle, identifies the act with the object, but the identification is through his unique theory of intentionality. The esse intentionale and esse natu- rale distinction is critically important. The object thought of and the thought are the same form exemplified in different subjects and different manners of existing. As Martin writes: ‘Thus when my thought has a match [adequatio] with the world, it is in virtue of two individualisations of the same form, in two different subjects, with two different manners of existence.’ Once again, the force of the intentionality thesis is paramount. This use of form with the possibility of the two modes of exemplification justifies both the ontological realism and the epistemological realism in Aquinas’s philosophy of mind. In this way, Aquinas sidesteps the classical problem with rep- resentationalism. What is known is the form of the object in the world, not a ‘copy’ of the form in the world. There is no ‘tertium quid’. Kenny often suggests that this is the important insight in Aquinas’s theory of intentionality. This is the basis, in addition, for Aquinas’s distinction between the ‘id quod’ and the ‘a quo’ aspects of sensation. The sensible species in the faculty is the ‘a quo’—the ‘that by which’ or ‘through which’ we sense something. It is not the object (the id quod) of direct sensation. Aquinas writes: ‘However, the sensible species or likeness [similitudo] is not what is perceived, but rather that by which the sense perceives (Summa Theologiae, I q. 85 a. 2, sed contra). He continues: ‘Hence, that by which the sight sees is the likeness of the visible thing [. . .] the likeness of a sensible thing is the form of the sense in act.’ The likeness—the sensible species (i.e. the similitudo)—is not the ‘id quod’—the ‘that which is known’ in sensation, but rather the ‘a quo’, the ‘that by which’ or ‘through which’ something is known.
In his De Veritate, Aquinas offers a precise distinction between the ‘a quo’ and the ‘id quod’ aspects of sensation: ‘It is by bodily sight that one sees a body itself; one does not see the likeness of the body. However, it must be noted that one sees through a likeness of the body’ (De Veritate II, q. 10 a. 8). The ‘id quod’ is the colour exemplified in the primary substance found in the external world. This colour is only sensed when the sense faculty has been impressed with the active power of the colour in the thing. The ‘impression, however, is not what is known; it is the means by which human per- ceivers are able to have sense knowledge of the world. It is through the sense impression in the faculty that the sense faculty ‘becomes’ the sense object in the external world, but immaterially or intentionally. The same form is exemplified ‘intentionally’ in the faculty and ‘existentially’ in the object; this is the Aristotelian insight further enhanced by Aquinas. There is an identity of form, one in esse intentionale and the other in esse naturale, indicating the two modes of exemplification utilized. Without this identity of structure rendered possible by the two modes of exemplification, the isomorphism of mind and reality in Aristotelian ontology and philosophy of mind would be impossible.
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