Home Economics Aquinas’s theory of perception: an analytic reconstruction
Two Senses of Intentio: From the Active Power to the Cognitive Faculty
In his causal theory of sensation, Aquinas’s account of intentionality is one of externalism. He holds for an ontological theory of intentionality, and the property of the possibility of achieving intentional states is a primitive in his ontology of the human person. A knower has an ontological capacity or ability, both the organ and the faculty, which are rooted in the substantial form of the knower; these abilities enable the knower to acquire forms of other things in an intentional manner. Also, the content of an awareness, what Aquinas refers to as esse intentionale, has a basic ‘tending towards’ or an ‘aboutness’ for an object in the external world. He writes: ‘An immaterial immutation is when a species is received in a sense organ or in a medium in the manner of an intention “per modum intentionis” ’ (Commentary on the Soul, bk II, 14). In commenting on this passage, Tellkamp suggested that it is important to understand the Arabian influences on Aquinas’s account of an intentionality theory. Tellkamp writes: ‘By saying that the species are being received in the medium per modum intentionis, he [Aquinas] broadens the scope of intentionality placing it outside the mind. In doing so he seems to follow mainly Avicenna’s theory of the intentio’.11 Yet Aquinas also writes that the most important characteristic of an intention is its direction towards an object, which is the property of ‘aboutness. In his Commentary on the Sentences, he writes: ‘The very name of an intention suggests that it is able to be received by moving towards a power; for it is said that to intend is a tending towards another item’ (‘inten- dere enim dicitur, quasi in aliud tendere’) (Commentary on the Sentences, II q. 1 a. 3). In the Prima Secundae of the Summa Theologiae, he writes much the same thing: ‘Intendere est in aliud tendere’ (Summa Theologiae, I—II q. 12 a. 5). We need only recall that in discussing sense knowledge, Aquinas uses explicitly the language of intentio: ‘In the operation of the senses, an immaterial change is required, through which an intention of the sensible form [intentio formae sensibilis] is received in the sense organ’ (Summa Theologiae, I q. 78 a. 3).
Aquinas appears to have at least two important senses of intentio, both of which are necessary in explicating his account of sense knowledge with the external senses. First, when the effect of a causal interaction with an entity in the external world produces an act of awareness. Second, The causally effective product in the medium that interacts with the sense organ and thus the sense faculty.
In the attempt to comprehend Aquinas’s theory of external sensation, the exact nature of this difference is difficult to spell out consistently. It was noted above that Tellkamp argues that in order to understand more precisely what Aquinas meant by intentio, one needs to consider his Arabic sources, especially Avicenna.  From the above accounts, it can be determined that Avicenna argues that the intentional being of a species has a mind-independent status in the medium together with an intentional existence in a sense faculty. This twofold account is significant in the explication of the production of an awareness of a sensible quality in a sense faculty.
It appears that there are two functions of the sensible species, which would be the form being transferred from the active power of the proper sensible in the primary substance to the sense faculty and sense organ in the human knower. This is, accordingly, the result of the active power of the proper sensible in the primary substance causing an awareness of the proper sensible in the knower. The active power produces an intentio, which affects the medium that exists between the proper sensible itself and the knowing organ and faculty. The medium is able to take on this form in an immaterial but not a cognitive mode of being. In other words, the medium does not ‘actualize’ the form of the active power, but it must exemplify the form in order for the intentio to ‘move’ from the active power to the cognitive organ and faculty. As Burnyeat argues, another primitive exists in Aquinas’s ontology of the sensing situation: the medium can ‘take on’ the form of the active power in a non-material manner. This is an ontological capacity of the medium. For Aquinas, it explains how the form transfers from the active power in the primary substance to the cognitive organ and faculty. This will be called ‘Intentio-I’. Once Intentio-I reaches the cognitive organ, it has some reaction there, which would be the intention functioning as an efficient cause. The cognitive faculty, then, is acted upon by the cognitive organ, and the result is an instance of formal cause, in which the cognitive faculty ‘takes on’ the form of the active power now rendered in an intentional mode (esse intentionale), which is a means by which the knowing person is aware of the proper sensible in the primary substance. This instance of an esse intentionale will be referred to as ‘Intentio-II. It is the means—the a quo—by which the perceiver is aware of the proper sensible of, say, the red in the Jonathan apple placed in the fruit bowl near the window—all of which would encompass the state of affairs existing externally from the perceiver. This is, in effect, the rather complicated explanation of how the knower is able to be aware of a proper sensible existing in the external world. This is Aquinas’s explanation of his externalism in sense knowledge. The three-term necessary relation helps us understand his reliabilism.
The notion of ‘some other way’ in referring to the causal analysis, therefore, has a twofold meaning for Aquinas. Explained more fully, this twofold interpretation is expressed by the two different categories of immutation: physiological or natural and intentional. The second sense, ‘immutatio spirituals’, is an indication of the basic non-material and non-reductionist ingredient in all intentional activity. Yet this is not the only sense of immutation that is necessary for visual sensation. In considering Democritus’s text above, Aquinas discussed Democritus’s version of a causal theory of perception. Accordingly, the ‘some other way’ needs to be related also to some type of physical causality. The problem here, therefore, is how to explain this physical causality in visual sensation without entailing materialism or physicalism. In his Thomas Aquinas on Human Nature, Pasnau argues that Aquinas’s theory of sensation is reducible to materialism:
I believe that Aquinas takes sensation to be a wholly bodily process. In saying this I do not mean to deny that sensation involves the soul and, more generally, formal causes; what I mean is that sensation involves the soul and other forms in a way that a modern materialist could readily welcome. That is, Aquinas thinks of sensation as an operation consisting entirely of various bodily parts undergoing change in various ways. There is no further, nonbodily or spiritual operation involved. Aquinas is what I call a semimaterialist, in that he believes some intentional states, and some forms of conscious experience, can have explanations that are, in our modern sense, wholly physical. This is a controversial claim, but I believe that the textual evidence is decisive.
Pasnau appears to dismiss the distinction between a ‘physical immutation’ and an ‘intentional immutation’. Often he appears to equate ‘immaterial’ with ‘spiritual’ rather than with ‘intentional’. Hence, while Aquinas does reduce his theory of sensation to materialism insofar as it is not Cartesian dualism, there is nonetheless an intentional- ity dimension here that is not reducible to physical causality. Pasnau’s analysis appears conflated on this point. Stump is correct when she writes the following about the reception of a form in a cognitive event: ‘Aquinas tends to use “immaterial”, “intentional”, and “spiritual” roughly synonymously to refer to this kind of change or reception of form.’ Nonetheless, if Pasnau argues that Aquinas is not a Cartesian in matters of sensation, then his analysis is in agreement with the tenor of the account proposed throughout this present book.
Proposing an adequate solution for this problem requires a return to Aquinas’s treatment of the proper sensible and the medium. In the case of visual sensation, colour is the proper sensible and the transparent diaphanum is the medium. In the Commentary, Aquinas suggested that the visible, which is colour, affects the transparent medium, which is the diaphanum. With visual sensation, the most obvious medium is air. Colour affects the medium, and the medium, as a physical body, reacts with the physical disposition of the eye. In other words, the medium ‘becomes coloured, and this physical body, which is a transporter (as it were) of colour, affects directly the physical organ of visual sensation, which is the eye. If this were a complete account of visual perception, then Hamlyn, Sorabji, and Burnyeat would be correct in associating Aquinas’s theory either with Democritus’s atomism or reductive materialism. However, any suggestion of identification is impossible structurally for the following two reasons.
First, the medium does affect the eye. However, it does not produce a corresponding physical immutation with the eye. In the following passage from his Commentary on the Metaphysics, Aquinas considers how the modification of organ and faculty occurs in the sensation.
And evidently sight is a more immaterial sense, if we consider the modification produced in it by its object. This is the case because all other sensible objects change both the organ and medium of a sense by a material modification; for example, the object of touch by heating and cooling, the object of taste by affecting the organ of taste with some flavour through the medium of saliva, the object of hearing by means of motion in the body, and the object of smell by means of the evaporation of volatile elements. But the object of sight changes the organ and medium of sight only by a immaterial modification; because neither the pupil of the eye nor the air becomes coloured, but these only receive the form of colour in a spiritual [intentional] mode of being. Therefore, because actual sensation [sensus in actu] consists in the actual modification of a sense by its object, it is evident that that sense which is changed in a more immaterial and spiritual [intentional] way is more spiritual in its operation. Hence sight judges about sensible objects in a more certain and perfect way than the other senses do. (Commentary on the Metaphysics, bk I, lec. I, sec. 6)
In considering the act of seeing, Aquinas writes: ‘solum objectum visus non immutat nec organum nec medium nisi spirituali immutatione’ (‘The object of sight informs neither the sense organ nor the medium by itself, but only through a spiritual [i.e. an intentional) “informing”.’) He continues this discussion in the following way:
Sight informs us of many differences between things, for we seem to know sensible things best by means of sight and touch, but especially by means of sight. The reason for this can be drawn from the fact that the other three senses perceive those accidents, which in a way flow from a sensible body and do not remain in it. Thus sound comes from a sensible body inasmuch as it flows away from it and does not remain in it. The same thing is true of the evaporation of volatile elements, with which and by which odor is diffused. But sight and hearing perceive those accidents, which remain in sensible, bodies, such as colour, warmth and coldness. Hence the judgment of sight and touch is extended to things themselves, whereas the judgement of hearing and smell is extended to those accidents, which flow from things and not to things themselves. It is for this reason that figure and size and the like, by which a sensible being itself is disposed, are perceived more by sight and touch than by the other senses. And they are perceived more by sight than by touch, both because sight knows more efficaciously, as has been pointed out, and also because quantity [magnitude] and those [accidents] which naturally follow from it, which are seen to be the common sensibles, are more closely related to the object of sight than to that of touch. This is clear from the fact that the object of sight belongs in some degree to every body having some quantity, whereas the object of touch does not. (Commentary on the Metaphysics, bk I, lec. 1, sec. 8)
It must be remembered that change for Aquinas is the reception of a form from another. If a physical reception were taking place with visual perception—i.e. when the coloured medium affects the eye—then the eye itself would become coloured. Obviously this does not happen, although Sorabji appears to say that it does for Aristotle. Accordingly, Aquinas suggests that ‘in some sense we find spiritual [intentional] immutation only, as in sight’ (Commentary on the Metaphysics, bk I, lec. 1, sec. 8). This means that the eye does not become red even though the coloured object and the medium both possess the causal power of red. Nonetheless, a certain physical reaction occurs. In other words, the coloured medium reacts with the disposed faculty for visual sensation. But the effect was not a ‘red’ eyeball but rather an intentional immutation, which will be what Aquinas refers to as the ‘sensible species’ Other senses also become changed physiologically as well as changed spiritually or intentionally. An example would be the sense of touch when it encounters a hot object. In this case, a physical change renders the sense receptor itself in the state of becoming hot. In the case of visual perception, however, only a spiritual or intentional immutation occurs. This intentional immutation is the basis for Aquinas’s claim that ‘immateriality’ is the root or basis for all knowledge; i.e. the ability of some ‘X’ to have the form of another without taking on its physical characteristics. In summation, therefore, the physical causality necessary for visual perception is the coloured medium affecting the visual disposition, which results in a spiritual or intentional immutation—i.e. the formation of the sensible species in the sense faculty of sight. In other words, Aquinas differs from Democritus because the physical reaction, which is the causal interaction from the objects in the external world, is not a sufficient condition for awareness to occur. It is not a sufficient but only a necessary condition. In addition, the intentional immutation, which is having the form immaterially in a cognitive state, which would be in a state of esse intentionale, is accomplished in the visual disposition, which is the sense faculty of sight. There are both material and cognitive elements in any sense awareness. The following passage illustrates the present points under discussion: ‘He remarks that colour-affected air itself modifies the pupil of the eye in a particular way, i.e. it imprints on it a likeness [the sensible species] of some colour, and that the pupil, so modified, acts upon the sensus communis. Similarly, our hearing [ . . . is . . . ] itself affected by the air’ (Commentary on the Soul, no. 773).
This discussion reaffirms the basic and rudimentary ‘immateriality’ of Aquinas’s thesis of intentionality, even on the level of sensation and perception. Because of this immaterial or intentional immutation, Aquinas cannot be classified as a strict atomist or materialist, as critics like Hamlyn and Sorabji are wont to do. One might suggest that Aquinas has atomist tendencies. But this appears reducible to nothing more than the claim that there are objects in the external world that affect causally our sense faculties resulting in sensation and perception. Aquinas writes: ‘the pupil of the eye is in potency with regard to all colours’ (Compendium of Theology, pt I, ch. 89). In effect, this atomist leaning is reducible to the fact that he is a realist who adopts a causal theory of sensation and perception. A truncated world exists for Aquinas, which world, when in the presence of a knower, causally affects those beings capable of intentionality—i.e. beings capable of having forms in esse intentionale. The material, physical world exerts a causal influence in the mental acts of perceptual awareness. However, these causal structures are not reducible to atomism. Rather, this is a further indication that Aquinas is an objective relativist in perception and, as Stump suggests and Jenkins concurs, a reliabilist with externalist leanings. Aquinas comments: ‘For the act of sensation is not an act of movement; rather to sense is to be moved; since, through the sensible object’s altering the condition of the senses in acting upon them, the animal [i.e. the perceiver] is made actually sentient from being only potentially so’ (Summa Contra Gentiles, bk II, ch. 82, n. 12).
Knowing, accordingly, is a species of qualitative and not quantitative change; furthermore, using Aristotelian categories, any act of knowing fits under the umbrella of alteration. Insofar as it is the actualization of a cognitive power, an act of knowing is reducible to a kind of ‘perfection’ of the agent, Aquinas writes that knowing ‘is a perfection of a knower qua knower; this is so because for something is known by a knower only insofar as the known is somehow in the knower’ (De Veritate II, 2). Aquinas lets his theoretical imagination run a bit wild in this next passage when he writes that insofar as what is known is in the knower as esse intentionale, ‘it is possible for the totality of the whole universe to exist in one thing’ (II, 2). This would appear to follow from the Aristotelian position that the intellect is, in a sense, all things. Parenthetically, this passage suggests the epistemological optimism of Aquinas in that he assumes that if there is a formally existing primary substance, it is capable of being known both by sense and by intellectual knowledge. Of course, this process of knowledge must begin with external sensation, which in turn depends on the exemplification of all three necessary conditions for an act of sensation to occur. This is an ‘in-principle claim’. The assumption appears to be that if an entity is a primary substance, it is capable ofbeing known. Of course, some of the postulations of contemporary physics would question this principle.
This interpretation of Aquinas’s account of sensation argues, therefore, for a three- term relation, which is at the core of the process for an act of awareness. However, this three-term relation, NC[O-M-F], is only the set of necessary conditions; it is not a sufficient condition for sensation. What happens with sight is that when the eye comes into contact with a coloured object in a properly lighted medium, then the faculty of sight is immuted or takes on the form of the object. This intentional immutation is the reception of the sensible species. What this amounts to is that the faculty of sight is now ready to see. But it cannot be overly stressed that the object of perception is the red object existing in the external world and not the sensible species itself. It is to stress this fact that Aquinas often reiterates passages like the following from his account of human nature in the Summa Theologiae: A conscious impression is related to a cognitive power as a medium; it is as form by which the faculty knows’ (Summa Theologiae, I q. 85 a. 2). ‘But the sensible species is not what is perceived, but rather that by which the sense perceives’ (I q. 82 a. 2 sed contra). ‘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’ (I q. 85 a. 2). ‘The likeness through which we understand is the species of the thing known in the knower’ (I q. 85 a. 8, ad 3; emphasis added).
Aquinas’s claim is that the exterior object affects the sense faculty in such a way that the faculty is now disposed to have sense knowledge of the object. This state of being actively disposed is when the faculty has the sensible species impressed upon its disposition. The perceptible object—the proper sensible—in the external world causes this sensible species. This sensible species is the effect of the physiological immutation but is expressed only by the intentional immutation. This point was developed in more detail by the Renaissance followers of Aquinas, most notably John of St Thomas (John Poinsot); these commentators write extensively about the species impressa. What they meant by this mildly convoluted term is that the sense faculty is now disposed properly to perceive a particular proper sensible. John of St Thomas writes the following: ‘Species autem impressa solum est id, quopotentia cognoscit tamquam principio, ut constat exD. Thoma’ (Ars Logica, pt II, bk II, q. 3).22 In referring to the rendering of the intellectus possibilis into Disposition-I, which renders it ready to know, John of St Thomas writes: ‘Species impressa est principium notitiae formalis; constituit enim intellectum in actu primo ad eliciendam notitiam formalem.’  And also in the following passage from the Ars Logica: At vero species impressa licet non sit primum principium tamen actuat potentiam ante cognitionem, et consequenter ante manifestationem actualem.’  These three passages suggest that the species impressa, which is used in both sense and intellectual knowledge, is that by means of which the knower is aware, not that which is the obj ect of knowledge. Later scholastic philosophers amplify this discussion by incorporating a species expressa at both the sense and intellectual realms. On the sense level, a species expressa is a phantasm formed both in the vis imaginativa and in the sense memory but never in the external sensorium. On the intellectual level, the species expressa will be the reflective concept formed in the intellectus possibilis.25
There is, however, a subtle play going on over the use of the terms, ‘properly disposed’. One resolution of this problem suggests that Aquinas, when referring to the sense faculties of the external sensorium, used ‘disposed to know’ in two ways, since he claims that visual perception is a sophisticated happening:
Aquinas suggests, therefore, a generic level of disposition and then a specific level of disposition. The generic level, Disposition-1/Actuality-2, is what distinguishes the faculty of seeing from the faculty of hearing. The category of proper sensibles in the external world is so constituted that the genus of colour is, as it were, structurally different from the genus of sound. But within this genus of each object of each external sense—the proper sensibles—there are further species. Thus the eye is not ordered only to ‘colour’, but to reds, greens, blues, and so forth. But in order to perceive these species of colour, additionally there must be an appropriate disposition. The reception of this disposition is caused ultimately by the thing in the external world through the physiological immuta- tion of the sense organ, which in turn results in a sensible species—the intentional immutation. This intentional immutation is the instance of esse intentionale of the structure of red in the external world, which in turn is an instance of esse naturale or esse reale. Accordingly, the sensible species is that which makes the faculty of vision or sight—the eye—disposed to sense this particular red rather than that particular green. That Aquinas is considering something like the species impressa of the Renaissance scholastics is modestly evident from several passages in the Summa Theologiae.
The first part of a passage noted earlier is important for the present discussion because Aquinas discusses the ‘change’, which is the intentional becoming of the sense faculty. The sense faculty is properly disposed already as an instance of Disposition-1/ Actuality-2 so that it might sense a member of a genus of a proper sensible. The second part refers to the formation of one type of phantasm in the vis imaginativa. In other words, Aquinas distinguishes between the sensible species received from the thing in the external world and the formation by the vis imaginativa of an ‘image’. These are two decidedly different functions of the sensorium:
The formation of the sensible species is a causal function only because there are proper sensibles existing in the external world. An image is neither identical to nor coextensive with a sensible species. This is another indication that Aquinas is not a representa- tionalist. In order for representative realism to be predicated of his theory of sensation, a ‘sensible species’ needs to be equivalent to an image. Aquinas denies, however, that such equivalence exists.
In conclusion, therefore, a process occurs rendering the disposition, which is an instance of Disposition-2/Actuality-1, disposed to do something in particular. This is the reception of the intentional immutation or the formation of the sensible species. This is accomplished by the sensible objects in the external world. Throughout this analysis, it must be remembered that the sensible species is not the direct object of sensation or awareness. It is the means by which (a quo) the sense apparatus is disposed so that the perceiver can be aware of the proper sensible (id quod) in the external world. The need for such immutation applies to all the faculties of the external senso- rium. One text in particular indicates this epistemological dependency.
At this point, a question arises about the mode of causal efficacy exercised by the medium and the proper sensible on the sense organ and sense faculty. In the passage just quoted, Aquinas claims explicitly that sight is not like sound, ‘for sound is caused by percussion and commotion of the air’ When colour affects the medium, there is not a causal process like sound. But it must be physical in some sense. Aquinas addresses this issue in the Commentary:
An indication of this is the fact that if a coloured body is placed upon the organ of sight it cannot be seen; for then there remains no transparent medium to be affected by the colour. The pupil of the eye is indeed some such medium, but, so long as the coloured body remains placed upon it, it lacks actual transparency. There must be a medium, for instance, air or something of the kind, which, being actualized by colour, itself acts upon the organ of sight as upon a body contiguous or continuous with itself. For bodies only affect one another through actual contact. (Commentary on the Soul, no. 432)
This is the only passage found in the Commentary in which Aquinas considers physical contact when discussing sight. However, he writes precious little about the exact workings of this physical, causal process. He continually writes in terms of forms acting on potentialities. The exact structure of this causal process, however, is left undetermined. Aristotle too said little about this important matter. This passage indicates that Aquinas is premodern in theory and does not understand the concept of ‘action at a distance, which was Newton’s contribution to physical theory. Aquinas demands physical contact in order to explain any causal efficacy in the material realm. This discussion reverts to the beginning of this chapter when a disposed sense organ (DO) was contrasted with a disposed sense faculty (DF).
One might argue that at this juncture in Western medieval philosophy, Aquinas like several others provided a metaphysical rather than a physical account of change. Often medieval philosophers confused or muddled the distinction between a physical inquiry and a metaphysical inquiry. Hence, often an attempt was undertaken to find ‘forms’ in much the same way that a physicist might attempt to find atoms. Put differently, there is a difference between the postulated entities used in metaphysics and the postulated entities used in physics. Hence, one might argue that Aquinas’s account is metaphysical rather than physical. On the other hand, the passage above from the Commentary indicates that actual physical contact occurs. Whatever the nature of this causality, in the texts Aquinas is disturbingly silent on this topic. Haldane emphasized that Aquinas’s causality is not reducible to efficient cause but is instead an instance of formal causality.
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