Causality of ‘Kind’ and Causality of ‘Mode’
Aquinas further explains this distinction between the specific causality exercised with the proper sensibles and the causality exercised by the common sensibles. He argues that there are two ways in which a sense faculty can ‘be acted upon’ or ‘affected’:
Now an object may affect the faculty’s immediate reaction in two ways. One way is with respect to the kind of agent causing this reaction; and in this way the immediate objects of sensation differentiate sense experience, inasmuch as one such object is colour, another sound, another white, another black, and so on. For the various kinds of stimulant of sensation are, in their actuality as such, precisely the special [proper] sense objects themselves. And to the proper sensibles, the sense faculty [as a whole] is by nature adapted; so that precisely by their differences is sensation itself differentiated. (Commentary on the Soul, no. 394)
This appears to be a type distinction. There is, however, another manner in which the object can act upon or affect the sense faculty:
On the other hand, there are objects that differentiate sensation with respect, not to the kind of agent, but to the mode of its activity. Insofar as sense qualities affect the senses corporeally and locally, they do so in different ways; for example, if they are qualities of large or small bodies or are diversely situated, i.e. near or far, or together or apart. Moreover, it is in this way that the common sensibles differentiate sensation. Obviously, size and position vary for all the five senses. And because they are not related to sensation as variations in the immediate factors, which bring the sense to act, they do not properly differentiate the sense faculties; they remain common to several faculties at once [hence ‘common sensibles’]. (no. 394)
Aquinas attempts to explicate the causal characteristics of both the proper and the common sensibles so that both might be classified as Sensation-Ia objects. He claims that what is directly perceived can affect, influence, or act upon the sense faculty or disposition in either of two ways. In the first, the affectation is according to the kind of agent which is doing the ‘affecting’. In the second, the affectation is not proportionate to the kind of agent but rather is directly related to the mode or manner in which the agent ‘affecting’ the sense faculty or disposition is found. In the first division, the kind of agent refers to the proper sensible. The coloured object as such—i.e. insofar as the thing has a colour-producing power—is considered as a kind of agent in that it reacts directly and proportionately with a sense faculty. In the case of sight, the object, which is colour, reacts in some causal way directly with the faculty of sight. The result is the mental awareness of seeing; this assumes, of course, that the other requisite conditions hold. The same structural account in terms of kinds or types is given for the other proper sensibles.
The common sensibles, on the other hand, do not affect the sense faculty directly as a specific kind of agent. Rather, their ‘affectation’ is in the manner or a way or mode of acting. Mode seems to be the manner in which a perceiver is aware of the common sensible. Thus, a perceiver can ‘see shape’ and ‘feel shape’. The mode seems to refer to a complex causal disposition, which is capable of affecting more than one faculty.
Aquinas is somewhat sketchy as to the positive analysis of how a mode functions with the common sensibles. Nonetheless, this kind/mode distinction may be an example of a token distinction rather than a type distinction.
In the Supplement to the Summa Theologiae, Thomas sorts out the difference between how a proper sensible affects the sense faculty and how a common sensible affects a sense faculty:
I suggest that a thing is perceptible to the senses of the body in two ways: (a) directly (as a kind), and (b) indirectly (as a mode). A thing is perceptible directly if it can act directly on the bodily senses. And a thing can act directly either (a) on sense as such or (b) on a particular sense as such. That which acts directly in this second way on a sense is called a proper sensible, for instance, colour in relation to the sight, and sound in relation to the hearing. Because as sense as such makes use of a bodily organ, nothing can be received therein except corporeally, since whatever is received into a thing is therein after the mode of the recipient. Hence, all sensibles act on the sense as such, according to their magnitude: and consequently magnitude and all its consequences, such as movement, rest, number, and the like, are called common sensibles, and yet they are direct objects of sense. (Summa Theologiae, Supp. to III, q. 92 a. 2; emphasis added)
In the Commentary, Aquinas discusses the role of magnitude in awareness:
In considering ‘the primary sensitive part’, Aristotle concludes about the organ of sense. Since from his teaching that sense receives forms into cognition immaterially (intentionally), which is true of the intellect also, one might be led to suppose that sense was an incorporeal faculty like the intellect; to preclude this error, Aristotle assigns to sense an organ, observing that the ‘primary sensitive part’, i.e. organ of sense, is that in which a power of this sort resides, namely, which is a capacity to receive forms without matter. For a sense organ, e.g. the eye, shares the same being with the faculty or power itself, though it differs in essence or definition, the faculty being as it were the form of the organ [ . . . ] Aristotle goes on to say ‘an extended magnitude, i.e. a bodily organ, is ‘what receives sensation’, i.e. is the subject of the sense faculty, as matter is subject of form; and yet the magnitude and the sensitivity or sense differ by definition, the sense being a certain ratio, i.e. proportion and form and capacity, of the magnitude. (Commentary on the Soul, no. 555)
Combining what is suggested in the Commentary about ‘kind’ and ‘mode’ together with the discussion of the role of ‘magnitude’ in the Supplement to the Summa Theologiae, one might propose that the function of ‘magnitude’ is the mode by which all sensibles act upon a sense organ. A sense organ is a bodily, ‘corporeal’ organ, and can be affected or acted on only with another bodily entity. Each sensible, proper or common, has a bodily, material component. Hence, all the sensibles are ‘common’. Yet there is a subset of sensibles that react directly and proportionately with a specific organ and faculty; these are the ‘proper or special sensibles. They affect an organ by the ‘kind’ of agent they are—colour, sound, heat, and so on. Put differently, the common sensibles are sensed only through the medium of the proper sensibles—colour, sound, etc. For Aquinas, the proper sensibles as active causal powers exist outside of the mind; hence red and blue and sweet and sour are not mind-dependent. Because these proper sensibles exist in matter, it follows that they exist in some way in a mode of quantity or magnitude. Accordingly, it is by colour, which exists as extended, that the common sensibles of shape and size are perceived.
The incidental obj ect of sense, however, works in a completely different manner. The second section of the above text from the Supplement to the Summa Theologiae considers this issue:
An indirect [incidental] object of sense is that which does not act on the sense, neither as sense nor as a particular sense, but is annexed to those things that act on sense directly. For instance, Socrates, the son of Diares, a friend, and the like which are the direct object of the intellect’s knowledge in the universal, but in the particular are the object of the vis cogitativa in human beings, and of the vis aestimativa in other animals. The external sense is said to perceive things of this kind, although indirectly, when the apprehensive power—the vis cogitativa (whose province it is to know directly this thing known), from that which is sensed directly, apprehends them at once and without any doubt or discourse (thus we see that a person is alive from the fact that he speaks); otherwise the sense is not said to perceive it even indirectly. (Commentary on the Soul, no. 555)
These passages explain the difference between the causality of the proper and of the common sensibles. Both are Sensation-Ia objects. The incidental object of sense is very different, and this is why it is a Sensation-Ib object.
Rather than providing a positive account of the structure of a mode and its corresponding function, other than as a ‘magnitude, which he should have done, Aquinas provides a twofold reductio ad absurdum argument indicating that the common sensibles and the proper sensibles cannot be identical. To begin, he argues that if the common sensibles directly affected the sense faculty as a kind of agent, this would structurally necessitate that a perceiver would need additional sense faculties in order to grasp each of the common sensibles. Aquinas then responds to this type of statement. In the first case, he argues that de facto a human perceiver only has five senses. If a perceiver is to apprehend directly the common sensibles as a ‘kind’ of agent, then this perceiver would have to possess as many faculties and organs as there are both proper and common sensibles. Aquinas here appeals to a ‘common sense position’ in that a human perceiver only has five external sense organs and faculties. This suggests, so he claims, that the common sensibles cannot be analysed structurally as causal powers in a manner identical with the proper sensibles. This is another instance of the naturalism in Aquinas’s philosophy of mind.
Secondly, Aquinas indicates that the common sensibles can be known by, or are the objects of awareness of, more than one sense. In effect, this is his principal reason for calling them ‘common sensibles’: they are ‘common’ to more than one sense. Appealing once again to ordinary sense experience, Aquinas suggests that a pre-analytic datum is accounted for by the common sensibles. In other words, human perceivers do in fact perceive the common sensibles with more than one sense; e.g. both the eye and the sense of touch are the means by which a perceiver determines the common sensible of shape. It follows, Aquinas suggests, that both the common and the proper sensibles cannot be direct objects of perception as a kind of agent. This would entail that only one sense faculty could know each of them. In the end, therefore, his distinction between ‘kind’ and ‘mode’ seems to be grounded in the fact that the proper sensibles are fitted to one sense faculty in the external sensorium while more than one faculty of this sensorium can perceive the common sensibles. Moreover, it appears that a mode can best be understood ontologically as a complex causal disposition, which is such that it can affect two or more different sense powers. This is in terms of magnitude. The ‘complex causal disposition, as an instance of magnitude, is the best analysis so far for unpacking Aquinas’s distinction between ‘mode’ and ‘kind’. Admittedly, this distinction and its explication by Aquinas are only modestly satisfactory. Nonetheless, it is his attempt to offer an explanation of the pre-analytic data about the external senses.
-  The interested reader might consult Timothy Suttor’s commentary, Summa Theologiae vol. 11(London: Eyre & Spottiswoode; New York: McGraw-Hill, 1968), (Ia q. 75-83), 134-5.