Home Economics Aquinas’s theory of perception: an analytic reconstruction
The Need for the Intellectus Agens
Accordingly, this structural difference between the nature of a sense faculty and the nature of the intellect provides Aquinas with the necessary ontological condition for postulating the intellectus agens. In the Summa Theologiae, Aquinas sums up nicely the differing relations of potency and act in a human person:
Sensible things are found in act outside the soul. And hence there is no need for an active sense. Wherefore it is clear that in the nutritive part of a human person, all the powers are active, whereas in the sensitive part all are passive. But in the intellectual part, there is something active and something passive. (Summa Theologiae, I q. 79, a. 3, ad 1)
In other words, the intellectus agens by the process of abstraction forms a species intelli- gibilis, which leads to the formation of a conceptus, which is the Disposition-2/ Actuality-1 state necessary for intellectual knowledge utilizing the intellectus possibilis. The intellectus agens is postulated because, in Aquinas’s ontology, essences, which are in some way the object of concept formation, do not exist reified as particular entities outside the mind. Aquinas rejects the possibility of subsisting universalia ante rem. Accordingly, there must be some means by which the mind can form concepts of these essential properties found in the individuals of a natural kind. On the intellectual level, therefore, the intellectus agens is that innate conceptual faculty by means of which a knower goes from Disposition-1 to a state of Disposition-2/Actuality-1. It is that faculty which ‘makes’ or forms conditions necessary for the acquired cognitive dispositions to be developed in the knower. The species intelligibilis, which is the ‘formal ability’ of a substantial form to be known, is similar in structure and function to the role of the species sensibilis, which is the ‘formal ability’ of a proper or common sensible to be known through the process of external sensation. Thus, the species intelligibilis is to the substantial form emmattered in a primary substance of a natural kind as the species sensibilis is to the active power of a proper or common sensible existing as an incidental form in the primary substance. The cognitive passive power of the sense faculty exists and is rendered active by the species sensibilis. Since the substantial form of the primary substance neither exists nor subsists as an essential property separated from the primary substance, an active faculty is needed to render the species intelligibilis which is existing intentionally in some way in potentiality in the inner sense of the sense memory active so that it may act upon the intellectus possibilis in order to form a concept. With the direct acquaintance of a universal form central to Platonic epistemology, there is no need for an intellectus agens. One must remember, however, that this ‘making’ of a species intelligibilis is different cognitively from ‘forming’ and ‘knowing’ a concept. The intellectus agens is a purely formal, innate structure that ‘abstracts.’ The intellectus possibilis—what Kenny perspicuously refers to as the ‘receptive’ intellect—is the cognitive faculty that ‘receives’ the species intelligibilis and then ‘knows’ the content of the concept.
Aquinas is cognizant of this causal function when he refers to the intellectus agens as a kind of ‘efficient cause’. In particular, when analysing the notion of the intellectus agens in the Summa Contra Gentiles, Aquinas often refers to it as an efficient cause. In the Second Book, Aquinas writes: ‘The other principle, having the role of efficient cause in the soul, “is the intellect by which all things are made”, and this is the intellectus agens’ (Summa Contra Gentiles, lib. 2 d. 78, n. 2); ‘There also is something, which, in the capacity of an efficient cause, makes all in act—and this is called the intellectus agens’ (n. 4), and ‘For Aristotle had already said that the intellectus agens is like an efficient cause . . . ’. (n. 8). It follows that whatever the precise analysis of the mental act of abstraction undertaken by the intellectus agens might be, it is not a simple act of direct acquaintance or intuitive apprehension common to twentieth century philosophers. In offering an analysis of the intellectus agens, Aristotelian philosophy of mind departs radically from its Platonic forebears. Furthermore, Aquinas rejects Plato’s theory of recollection, which depends upon the mind’s being ‘directly acquainted with’ the forms of objects prior to birth.
The present account, in addition, reiterates Aquinas’s concerns about the Platonic distinction between opinion and knowledge. Accordingly, it is natural and quite easy for a perceiver to have perceptions. The life of the senses, however, can keep one within the confines of the Platonic ‘cave’ discussed in the Seventh Book of the Republic; it is difficult to acquire knowledge of concepts in the brilliance of the light of ‘truth’ beyond the trappings of the cave; ‘. . . as a subject moves from primary potency into primary actuality when it acquires knowledge through teaching, so too a subject’s primary potency to the possession of a sense faculty is actualized by her birth’ (Commentary on the Soul, #373). In other words, insofar as a human knower has functioning sense organs and faculties, i.e., innate Dispositions-1/Actualities-2, she is guaranteed perceptions. In concept formation, on the other hand, these Dispositions-2/Actualities-1 ‘have to be acquired through application and discipline’. Whereas sense knowledge is easily attainable, intellectual knowledge requires a contribution on the part of the agent. With the intellectus agens as a necessary condition for concept formation, Aquinas, much like Brentano, opts for a structured mental act position. This dimension of his philosophy of mind will be discussed later in this text.
Once Aquinas distinguished between potency and act in both conception and perception, he next considers the difference between the actual process of sensing and the actual process of thinking:
Aristotle sets himself out to discriminate between actual sensations and thinking. And he finds the first reason for distinguishing these activities in the difference between their objects, i.e., the sense objects and the intelligible objects, which are attained by actual sensation, and thinking respectively. The sense objects, which actuate sensitive activities—the visible, the audible, etc.—exist outside the soul. The reason is that actual sensation attains to the individual things that exist externally.
On the other hand, rational knowledge is of universals (essences), which exists somehow within the soul. Whence it is clear that the person who already has scientific knowledge about certain things does not need to seek such things outside of herself. Such a knower already has them inwardly, and is able, unless prevented by some incidental cause, to reflect on them whenever one pleases. But a person cannot sense whenever one pleases; not possessing sense objects inwardly, one is forced to receive them from the outside. (Commentary on the Soul, #375)
In this passage, Aquinas considers what in De Ente et Essentia he refers to as knowledge of essences rather than knowledge of universals. The awareness of an essence is a ‘first intention’ whose content is in principle isomorphic with the set of dispositional properties which comprise a substantial form instantiated with a piece of prime matter to form a concretum or primary substance. The awareness of a universal is a ‘second intention’ whose content is a mental relation produced by the mind in reflecting upon the content of a first intention. This is the relation of ‘one to many’, which is what Aquinas calls the universal. These two passages are lucid expositions of the dichotomy Aquinas accepts in regard to different objects of knowledge. The sense faculties are the means by which a perceiver is aware of the proper and common sensibles found in the individual concreta of the external world; however, the awareness of the individual concretum itself as a primary substance is not part of this discussion. The intellectual faculties, on the other hand, are the means by which the knower is aware of two kinds of concepts:
The intentional objects of both acts of the intellect, the first and the second intentions, are mental existents. Neither exists as concreta in the external world. Through this distinction between sense knowledge and intellectual knowledge, Aquinas adopts the Platonic dichotomy between objects of knowledge. His analysis of the objects of knowledge, however, is far from Platonic. This present chapter suggests how this bypassing of Platonism occurs. Jacobs perceptively put the matter this way; for Aquinas ‘. . . the making of concepts is an activity occurring in natural beings and on account of their relation to and interactions with things in the world’.
So far, very little has been said about the objects of sensation and perception. The next chapter begins the discussion of these objects of sensation. The latter chapters of the book, beginning with an analysis of the vis cogitativa, will provide an analysis of the objects of perception. It is through this distinction between distinct objects of sensation and perception that Aquinas’s account of perception transcends the limits of classical British empiricism. Sensation, for Aquinas, is the awareness of what he, following Aristotle, calls the proper and the common sensibles. These are the colours, sounds, tastes, shapes, figures, and so forth, of sense knowledge. Perception, on the other hand, is analysed in terms of the awareness on the sense level of an individual as an individual. This awareness of an individual is beyond the limits of Berkeley and Hume, both of whom reduce an individual more or less to a collection of sensible qualities. Like Thomas Reid, Thomas Aquinas affirms the distinction between sensation and perception. This distinction will be the principal topic for the discussion of the vis cogitativa later in this book.
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