Home Economics Aquinas’s theory of perception: an analytic reconstruction
Principle D. An act remains ‘specifically’ the same but it may have different embodiments or exemplifications in different potencies
This principle asserts that the ‘species’—i. e. the content—of an act remains the same even though different existential instances or embodiments of the species or natural kind exist. All existential instances are different numerically from one another but remain specifically the same. This is Aquinas’s way of suggesting a solution to the perennial ‘one-many’ problem. The basic sameness of things grouped together in natural kinds is accounted for because members of each group possess substantial forms identical in content. Individuality or particularity, on the other hand, occurs insofar as these forms, which are specifically the same, have been instantiated in different potencies. An entailment of this principle is that the ‘matter’ or ‘potency’ of any given thing is that which accounts for its individuality. In De Ente et Essentia, Aquinas writes: ‘The principle of distinct individuality is not matter in any and every sense of the word, but only marked-off matter. By marked-off matter I mean matter thought of as having definite dimensions’ (De Ente et Essentia, lec. 2).
The principle of distinct individuality is what Aquinas refers to as ‘materia signata quantitate’. Furthermore, the individuating principle requires not merely that matter be ‘marked off’ but that it is ‘properly disposed’. What is properly disposed can receive the same type or species of form, i.e. forms of the same species. Therefore, an act, while remaining specifically the same as to content, can be found in numerically different potencies. Principle D refers also to incidental change. A primary substance might be the subject of changing incidental qualities, with the subject itself remaining the same. In this case, the incidental quality could be specifically the same yet be instantiated in several primary substances, which are different numerically.
Principle D also expands the significance of Principle A. Taken with Principle A, the previous discussion makes it clear that an ontological act—either a substantial form or an incidental form—can never be an ontological existent by itself. Any awareness of this kind of isolated act must be a mental construct. If the act were not a construct but had existence by itself, then it would follow that Aquinas is a Platonist; this would entail that Aquinas embraced some form of extreme realism—universalia ante rem, either in rerum natura or in a subsistent transcendental realm—which he denies as holding in the natural world. Parenthetically, this is why Aquinas postulated the intel- lectus agens as a necessary condition for abstraction in preparation for concept formation. However, this construct of the form is not a fiction. Rather, in the case of substantial form, it is the ontological ground for establishing the necessity of a counterfactual proposition. The above analysis elucidates part of the cash value of the often- used category of ‘moderate realism’ when referring to Aristotelian ontology.
Principle D-1. A knower is, by definition, any ‘X’ which has a set of dispositional properties to acquire or to exemplify acts in a non-entitative or non-materialist manner
This principle is a corollary, albeit an extremely important one, of Principle D. In Brentano’s terminology, this corollary is a description of the fundamental principle elucidating Aquinas’s thesis of intentionality. This is the case if one considers an inten- tionality thesis as the philosophical description distinguishing ontologically a knower from a non-knower. Principle D-1, therefore, indicates the structural difference between knowers and non-knowers. In Aquinas’s ontology, this principle establishes the basis of intentionality. Principle D-1 asserts that a knower has cognitive potencies or dispositions to receive or to be impressed with acts in a special manner or mode. This special manner or mode entails that a knower, when receiving or exemplifying an intentional form, neither literally nor entitatively becomes the type of thing which the knower knows. Accordingly, a knower is, by definition, an existent that possesses a peculiar set of dispositional cognitive properties enabling it to take on the form of the material thing ‘immaterially’. This immaterial reception of a form implies that a distinct object with formal existence (esse naturale) does not come about from the informing of a cognitive power or potency: on the contrary, this results in an esse inten- tionale. In this context, it is necessary that the concept of ‘immateriality’ not be misunderstood. It does not refer to a spiritual entity akin to a Cartesian ‘res cogitans’, which is an immaterial substance. Rather, it refers to a knowers possession of the requisite set of dispositional properties enabling that particular knower to acquire acts or forms without the act or form becoming existentially ‘tied down’ with a material potency.
That Aquinas regards knowledge faculties as cognitive potencies or dispositions is expressed in the following passage: ‘The senses and the intellect are distinguished from the sensible and the intelligible objects in that both the senses and the intellect are in the state of potentiality (Summa Theologiae, I q. 14 a. 2; emphasis added). In a general sense, therefore, there are two modes for the reception or exemplification of forms into potencies: (a) materially or entitatively and (b) immaterially or intentionally. An entitative reception of a form, either a substantial form or an incidental form, entails a new instantiation of a physical existent. The result is an entity that has esse naturale. It is either a new kind of a thing or a thing with a new incidental property. An immaterial reception of a form, on the other hand, entails the instantiation of a ‘piece of knowing’. The result is an act of awareness that has an esse intentionale. Aquinas discusses a twofold division of ‘immutation’ in the reception of forms; this division corresponds to the reception of forms entailing either an esse naturale or an esse intentionale.
There are two kinds of immutation or change, natural and intentional. It is natural change when the form of the source of change is received into the subject of the change in a physical manner. An example would be heat as it is absorbed by the object being heated. It is an intentional change when the form of the source of change is received in the subject of change in an immaterial manner. An example would be the manner in which the form of color is in the eye. The eye does not become physically the color it sees. The activity of the senses involves immaterial reception of forms. In this way, the intention of the sensed form comes to be in the sense organ. (Summa Theologiae, I q. 78 a. 3)
In the Supplement to the Tertia Pars of the Summa Theologiae, Aquinas explains more fully this distinction regarding the two kinds of immutation or alteration:
However, it must be observed that things outside of the soul in two ways affect (immutat) the organs of the senses:
Accordingly, the first reception does not cause sensation, properly speaking, because the senses are receptive of species in matter but without matter, which is to say, without the material ‘being’ which the species had outside the soul. (De Anima, ii lec. 121)
This reception affects the nature of the recipient, because in this way the quality is received according to its material ‘being’. (Summa Theologiae, Supp. to III, q. 82 a. 3)
Aquinas explicitly uses the term ‘intentio in the above passage. In his response to the second objection to the above text, Aquinas comments: ‘inanimate bodies are altered by sensible qualities only naturally and not spiritually (i.e. intentionally)’ (Summa Theologiae, Supp. to III, q. 82 a. 3 ad 2).
Haldane writes about the esse intentionale and esse naturale distinction as follows:
Each actuality (thought and object) has a structuring principle (concept and substantial form); and these principles, though distinct in the modes of their actualization, are specifically alike. The form of dog exists naturally and substantially (in esse naturale) in the dog, and intentionally and predicatively (in esse intentionale) in the thought.
The important insight that Haldane offers is that in intentionality theory, the intentional form is in a predicative mode exemplifying the content of the substantial form, but in a different manner. It is not a substantive mode and thus not an instantiation of this form. It follows that in Aquinas’s account, there are non-empirical modes for the reception of forms. Furthermore, in his Commentary on Aristotle’s On the Soul, Aquinas asserts unequivocally that the ‘receiving of a form without matter’ is the ontological ground for distinguishing a being with only esse naturale from a being capable of possessing an esse intentionale. This denotes the ontological ground for intentional- ity. Here, Aquinas elaborates upon this twofold manner of receiving forms with an explicit reference to the esse intentionale involved in sense perception.
Every potency receives something from the agent insofar as it is an agent. Yet an agent acts through its form, and not through its matter. Therefore, every potency receives a form without matter.
Sometimes, however, a form is received into a potency according to a different mode of being than that found in the agent. . . . In this way, the senses receive form without matter, because the form has a different mode of being in sense knowledge than in a physical object. For in a physical object, the form has natural being (esse naturale) while in sense knowledge, it has intentional being (esse intentionale). (Commentary on the Soul, no. 553)
This distinction between esse naturale and esse intentionale entails that in Aquinas’s ontology, there are at least two generic classes of things—knowers and non-knowers. Non-knowers are grouped into real classes—specific ontological categories—insofar as a group of existents—water, sodium chloride, poplar trees, acorns, and so forth— possess the same kind of substantial form. In other words, a class or natural kind of thing is determined insofar as each member of that class or kind has the same set of specific, sortal dispositional properties. This follows from an earlier suggestion that a substantial form in Thomas’s ontology is best analysed in terms of the ontological ground for a supreme set of dispositional properties determining a natural kind of specific objects.
Each distinct group of knowers possesses in its substantial form a supreme set of dispositions, which might be referred to as ‘a primary integrated complex of dispositions’. This formal structure places the knower into a specific natural kind—e.g. cats, horses, sheepdogs, human beings. The class-determining set of dispositional properties in knowers, however, contains one additional set of dispositional properties which non-knowers lack. This additional set of dispositional properties comprises the cognitive abilities to receive forms in a non-entitative manner. Hence, the ability to receive or exemplify forms in a non-entitative manner as instances of esse intentionale—both through sense knowledge and intellectual knowledge—spells out the ontological force of Principle D-1.
Aquinas argues that not every thing possessing an Aristotelian ‘soul’ can have knowledge:
Aristotle’s analysis also gives us the answer to another question, namely, why plants do not feel, though they have some share in soul and are affected by certain sense-objects; i.e., tangible things, as well as by heat and cold. The reason why they do not feel is that they lack the proportion needed for sensation, in particular that balance between extremes of the tangible qualities which is a prerequisite of the organ of touch, apart from which there can be no sensation. Hence, they have no intrinsic principle for receiving forms ‘apart from matter. This means that they have no sense. They are affected and undergo changes only materially. (Commentary on the Soul, no. 557)
Only some individual natures have substantial forms possessing dispositional properties constituted as cognitive powers. Some of these powers are connected with bodily organs. This set comprises the external and the internal senses. In human knowers, another power exists independently of any set of bodily powers, and this would be the intellect, both active (intellectus agens) and possible (intellectuspossibilis) for Aquinas.
By way of summary, the following passage indicates that an additional set of dispositional properties establishes the ontological difference between knowers and non-knowers:
Knowing agents differ from those that do not know because non-knowers possess their own form only. On the other hand, the knower is adapted from its origin to possess the form of another thing. This means that the species of the known thing may be present in the knower.
The nature of a non-knower is more restricted and limited, while the nature of a knower has greater fullness and extension. This is why the Philosopher claims in the Third Book of On the Soul that ‘the soul is in a way all these things. (Summa Theologiae, I q. 14 a. 1)
The above collection of texts establishes the importance Aquinas gave to the concept of immateriality as the characteristic property grounding his thesis of intentionality. In his Commentary on the Soul, Aquinas elucidates this concept of immateriality in terms of a set of dispositional properties which places an individual into a particular class— which is obviously the class of knowers:
We speak [. . .] in one sense of a potency when we say that a human person is a knower. This refers to the person’s natural capacity for knowledge. A human being, we say, is one of that class of beings that know or have knowledge, meaning this, that a person’s human nature can know and form habits of knowing. (Commentary on the Soul, #359)
A human person is said to be ‘able’ (to know) through belonging to a certain genus or ‘matter’; that is, one’s human nature has a certain disposition that puts the person in this genus, and the person as knower is in potency to knowledge as matter is to form. (Commentary on the Soul, no. 360)
In the last passage, ‘matter’ refers to ‘second matter’; this concept of second matter— also what Aristotle refers to as ‘secondary substance’—is best understood as the essence of a primary substance.
Aquinas’s characterization of intentionality, moreover, is not merely as an ordinary dispositional property. Rather, the built-in characteristic of ‘tending towards’ or ‘aboutness’ is part of the ontological structure of this dispositional property. Aquinas stresses continually that all knowledge implies that the thing known is somehow present in the knower. Accordingly, the set of dispositional properties, which constitutes a knower as a knower, enables the knower to go beyond itself and yet not physically and entitatively become that which it is ‘tending towards’. In Aquinas’s ontology, therefore, immateriality is the ontological dimension grounding the possibility of inten- tionality. In discussing Empedocles’s theory of perception in his Commentary on the Soul, Aquinas offers additional analysis of the claims of immateriality and the basic ‘tending towards’ property characteristic of mental acts. Empedocles held as his principal epistemological principle: ‘Like knows like.’ Furthermore, Empedocles provided a literal interpretation of this principle. An entailment of this principle is that the sense faculties are constituted entitatively of the same kinds of object that exist in the material world. According to Empedocles, the elements, which make up the sense faculties and enable perception to occur, are the same structurally and entitatively as the constituents of the physical objects in the external world. In commenting upon the Empedoclean principle and its implications for the philosophy of mind, Aquinas spells out his own thesis about knowledge:
note that all, who, like Empedocles, said that like was known by like, thought that the senses were actually sense objects—that the sensitive soul was able to know all sense objects because it consisted somehow of those objects; that is, of the elements of which the latter are composed.
Two things follow from this hypothesis put forward by Empedocles:
(a) If the senses actually are, or are made up of, the sense objects, then, if the latter can be sensed, the senses themselves can be sensed.
(b) Since the presence of its object actually exists in the faculty of sense as part of its composition, it follows that perception can take place in the absence of external objects.
But both of these consequences are false. (Commentary on the Soul, nos 352, 353)
In refuting Empedocles’s position, Aquinas in effect ruled out any theory of physical- ism or reductive materialism—positions that Thomas ascribes to the ancient naturalists (‘antiqui naturales’) (Summa Theologiae, Ia q. 75 a. 1 ad 2). In Aquinas’s theory, neither Cartesian substance dualism nor reductive materialism is compatible with a thesis of intentionality. Texts in both the Summa Theologiae and the Commentary assist in this analysis of intentionality.
In contemporary philosophy, Haldane argues against a physicalist account of Aquinas’s theory of intentionality: ‘Thus, in so far as Physicalism is committed to the possibility of a descriptively adequate extensionalist theory of human beings, the ineliminabilty of intentional characterisations is problematic.’ This is not an ersatz issue in Aquinas’s theory because Sorabji argues for a physicalist account of intentionality in Aristotle and theories based upon the Aristotelian theory of sense and mind. In addition, Cohen argued that for Aquinas, the immaterial reception of a form in sensation is a reductively physical process. The value of the above passage does not lie in the arguments Aquinas provides attempting to refute representationalism. Rather, their philosophical import lies in what is affirmed within the passage. If ‘like knows like’ is an adequate philosophy of mind principle, then it must be understood in a manner quite different from the analysis put forward by Empedocles. In effect, Aquinas affirms the following two propositions: (a) the centrality of an immaterial or intentional reception of forms—point number one in the above passage; (b) a basic ‘tending towards’ or ‘aboutness’ property to an object beyond the knower itself—point number two.
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