Objects of Sensation
In his discussion of the objects of sensation, Aquinas follows the threefold Aristotelian division: (a) the proper or special sensibles; (b) the common sensibles; (c) the incidental objects of sense. In every classification where Aquinas considers sense knowledge, he adopts this threefold division of sense objects: ‘Now the term sense-object is used in three ways, one way incidentally [per accidens] and in two ways essentially or absolutely [per se]. Of the latter, we use one if referring to the special objects proper to each sense, and the other in referring to the objects that are common to more than one sense in all sentient beings’ (Commentary on the Soul, no. 383). Aquinas claims that the objects of sensation are not to be rendered significant in a univocal way. On the contrary, there appears to be a type of a hierarchy for these objects of sense knowledge; this hierarchical classification will become clearer as the present discussion unfolds. For Aquinas, there are two generic kinds of sensible objects:
- (a) those that are perceived directly—the proper sensibles and the common sensibles.
- (b) those that are perceived only in conjunction with the directly perceivable sensibles—the incidental object of sense. In other words, one perceives the ‘son of Diares’ (an incidental object of sense) directly, but qua coloured object since the son of Diares is neither a proper nor a common sensible.
From the outset, one begins to notice a difference between Aristotle and Aquinas on the one hand and the classical British empiricists on the other, especially on the matter of the sense object that is ‘perceived indirectly.
In order to provide a fuller development of the various categories of objects of sensation, several passages from the Commentary require discussion and analysis. Aquinas first considers the ‘proper’ or what are sometimes called the ‘special’ sensibles:
Aristotle explains the members of the division, and first what he means by a special [proper] sense object. He says that he means by this term what is perceived by one sense and by no other, and in respect of which the perceiving sense cannot err. Accordingly, it is proper to sight to know colour, to hearing to know sound, to taste to know flavour or savour. Touch, however, has several objects proper to itself: heat and moisture, cold and dryness, the heavy and the light, etc. Each sense judges the objects proper to itself and is not mistaken about these, e.g. sight with regard to such and such a colour or hearing with regard to sound. (Commentary on the Soul, no. 384)
In the Quaestiones Quodlibetales, Aquinas writes about the relation between the sensible object and the awareness of that object: ‘External sense knowledge [cognition] is attained solely by the modification of the sense faculty by the sensible. Therefore, it is by means of the form which is impressed by the sensible object that sensation takes place’ (Quaestiones Quodlibetales V, q. 5 a. 2 ad 3). This passage firmly claims that externalism is central to Aquinas’s account.
Aquinas next considers the common sensibles:
Considering the second member of the division, he remarks that the common sense objects are five: movement, rest, number, shape and size. These are not proper to any one sense but are common to all. We must not take this to mean that all these are common to all the senses, but that some of them, i.e. number, movement, and rest are common to all. Touch and sight, however, perceive all five. It is clear now what are the sense-objects that are such in themselves or absolutely. (Commentary on the Soul, no. 386)
Thirdly, what will become Aristotle and Aquinas’s unique contribution to the philosophy of perception, the incidental object of sense, is discussed: ‘when the likeness of a human person is in sight; she is not there because she is a human person, but because she is a coloured object [sed in quantum huic colorata accidit esse hominem]’ (Summa Theologiae, Ia q. 17 a. 2).
In these passages, ‘sense’ is used to refer both to the external senses, when considering the proper and the common sensibles, and to the internal senses, when discussing the incidental object of sense. The internal senses, in addition to the vis cogitativa, are the imagination (vis imaginativa) and the sense memory (vis memorativa). Hence, the internal sense faculties have a more complex function than providing an awareness only of the incidental object of sense. Accordingly, Aquinas distinguished three generic kinds of objects of perception: (a) the proper or special sensibles; (b) the common sen- sibles; (c) the incidental objects of sense.
In his consideration of the proper sensibles, Pasnau remarks that Aquinas does not reduce this sense object to the category of secondary qualities: he notes correctly, ‘most such strategies would not have been acceptable to Aquinas.’ In principle, Aquinas rejects all sense data accounts, because the objects of sensation for him are identified with external objects. Pasnau continues this analysis with the following remarks, with which Kenny and Putnam would agree: ‘Aquinas resists appealing to inner conscious experience. The position Aquinas instead embraced is that the primary (proper) sensibles are basic and objective features of the external world, irreducible to quantifiable properties (the common sensibles) or to anything else.’ These passages suggest again the thrust of ontological realism and epistemological realism with which Aquinas articulates his philosophy of mind. A modern empiricist certainly Aquinas is not! In discussing the significance of Aquinas’s realism, Haldane comments: ‘[This] is a serious attempt to develop a philosophical theory of cognitive psychology, consistent with the assumption of epistemological realism; and [it] offers important insights for those who would attempt such a task today.’ The thrust of this study is congruent with Haldane’s suggestion.
In addition, Aquinas introduces an important distinction: (a) the external senses and the external sensorium; and (b) the internal senses and the internal sensorium. The external and internal sense division is based upon the physiological locus of the sense organ in question—is it in the extremities of the body or it is ‘in the head, as it were? The external and internal sensorium dichotomy depends on whether the sense faculty requires a phantasm in order to function. The internal sense faculty of the sensus communis will not require a phantasm; hence, it is an internal sense faculty that is part of the external sensorium.
Aquinas argues furthermore that the internal sensorium—the imagination, the vis cogitativa, and the sense memory—has a distinct set of functions and objects. He dis?tinguishes several types of mental activity, each of which is peculiar and distinct to itself. Each mental act will depend on a particular kind of object. Regarding the activities of the internal sensorium, Aquinas offers the ‘phantasm’ as the vehicle by which mental acts of the internal sensorium are distinguished from the mental acts of the external sensorium. The introduction of phantasms further complicates and expands an already complex structure of sense knowledge. Accordingly, the rather simplistic sense data position on perception is opposed structurally to Aquinas’s theory.
-  Aquinas is more complex in discussing objects of perception than are e.g. Moore and other early analytic philosophers, who wrote about sense data constituting the only category for an object of sensation.British empiricists like Berkeley, Hume, and Mill also appear to have one general category for sensequalities.
-  Pasnau, to the contrary, reduces Aquinas’s perception theory into a ‘third thing’ or ‘representationalist’position; he argues that in addition to a knowing disposition or power and a suitable external object, eachmental act of sense knowledge for Aquinas requires an intentional species, which Pasnau suggests is a ‘ter-tium quid’ between the mental act and the object known. See Robert Pasnau, Cognitive Theory in the LaterMiddle Ages (Cambridge: Cambridge University Press, 1997). There are two responses to Pasnau’s claimforcing Aquinas into representationalism. (a) Aquinas adopts the ‘id quod’ versus the ‘a quo’ distinction,suggesting that the object of knowledge—what is known—is distinct from the means by which the object isknown. The various species in direct perception are ‘a quo’ epistemological means and not ‘id quod’ objectsof knowing. (b) John P. O’Callaghan offers the following retort to Pasnau: Aquinas distinguishes betweenefficient cause and formal cause. Pasnau appears to reduce a cognitive species to an efficient cause whereasit is, according to Aquinas, a formal cause. In discussing concept formation and knowing, O’Callaghanwrites: ‘We conceive, and in our conceiving we grasp things other than our conceiving’: Thomistic Realismand the Linguistic Turn (Notre Dame, Ind.: University of Notre Dame Press, 2002), 169; O’Callaghan furthersuggests that Pasnau is confused because he reads Aquinas through Ockham’s eyes. An analysis of the differing roles played by efficient and formal causes in Aquinas’s account occurs later.
-  Pasnau, Cognitive Theory, 185. 2 Ibid.
-  9 Pasnau, however, proposes a role for a (tertium quid' in every mental act. It is unclear how this direc
-  tion towards even a modest form of representationalism squares with externalism.
-  John Haldane, ‘Aquinas on Sense-Perception, Philosophical Review 92(2) (1983), 234.