The Sensus Communis and the External Sensorium
In order to complete the analysis of the mental act of the sensus communis, one final point needs to be considered. Aquinas includes this faculty among his listing of the internal senses: ‘So there is no need to assign more than four internal powers of the sensitive part—namely, the sensus communis, the imagination, and the aestimative (or the vis cogitativa in human beings) and the memorative powers’ (Summa Theologiae, I q. 87 a. 4). The reason for placing the sensus communis as an internal sense faculty appears to be based on Aquinas’s physiology, where the bodily organ in which the sensus communis is found was located somewhere in the brain. With this physiological location, obviously it could not be classified as one of the external sense organs. Structurally, however, the sensus communis is part of the external sensorium. The object of the sensus communis is not an image, phantasm, or any type of post-sensation, ter- tium quid mental entity. The object of the sensus communis is a concrete whole unified from the discrete data of the external senses. Textually, Aquinas concludes his discussion of the sensus communis in the Commentary with the following remarks: ‘Concluding, Aristotle says that he has now discussed the principle according to which an animal is said to have, or be able to have, sensations’ (Commentary on the Soul, no. 614). Aquinas suggests an important distinction between the following categories: (a) the external senses and the internal senses, and (b) the external sensorium and the internal sensorium.
The distinguishing factor behind the external and internal senses appears to be the physiological location of the organs of sensation. Each organ of the external senses is located on or near the immediate surface of the body. The internal senses, on the other hand, have their organs located within the body, presumably in the brain. In De Veritate, Aquinas writes: ‘the organs of the cognitive powers of imagination, of memory, and of the vis cogitativa, are in the brain itself, which is the place of greatest moistness in the human body’ (De Veritate II, q. 18 no. 5). The division between internal and external sensoria, on the other hand, is determined by means of the function of the corresponding mental acts. The function of the external sensorium is to be aware of the objects in the external world, which are present immediately to the perceiver in a causally efficacious way. If there are no objects, then there will be no mental acts with the external sensorium. Of course, the object is reducible to the collection of proper and common sensibles known by the respective external senses and combined into a perceptible unity by the sensus communis. In discussing the need for sense organs for inner sense in Thomas, Kenny writes: ‘The inner senses, he thought, were like the outer senses in having bodily organs: only, the organs of the inner senses were inside the body and not at the surface. Thus, the organ of the [vis] cogitativa is “the middle cell of the head” and the fancy (imagination) has an organ that is injured in cases of seizure or coma.’ The internal sensorium, on the other hand, has the ability to both ‘remember’ what has been perceived and to ‘interpret’ what is currently experienced. Both of these functions go beyond the immediate data of the external sensorium. What distinguishes the internal from the external sensorium is the presence of phantasms. In Aquinas’s philosophy of mind, the role of phantasms is a necessary condition for the intentional activities of inner sense.
From the fact that the sensus communis is the root of consciousness, it follows that Aquinas would claim that sleep results because the sensus communis is affected and is not working. The physiological claims are, of course, not relevant here. What is relevant, however, is the consequence of placing the sensus communis with the external sensorium. Since this is the case, it follows that neither images nor phantasms are used with the sensus communis. It appears that those historians of philosophy who suggest that a phantasm is the direct object of the sensus communis fail to reconcile the ramifications of the consequent representationalism with Aquinas’s strong assertion of direct realism associated with his externalism.
The final necessary condition for sense perception is the working of the sensus communis. In the previous chapter, the necessary conditions for the occurrence of a mental act for an external sense were explicated in terms of a necessary triadic relation, NC[O- M-F]. In addition to this triadic relation, the final necessary condition is the function of the sensus communis. In other words, the sufficient condition for sensation is the occurrence of a series of necessary triadic relations—proper and/or common sensibles, adequate mediums, and properly disposed sense organs and faculties—together with the mental act of the sensus communis, the root of consciousness. When this occurs, one is aware of sensible qualities existing in the external world. In this way, Aquinas affirms his ontological realism and his epistemological realism, which are the ontic grounds for his externalism.
In conclusion, Aquinas’s account of sensation is a conjunction of a series of necessary three-term relations together with the mental act of the internal sense faculty of the sensus communis. The final state of sensation by means of the external sensorium is the awareness of a concrete whole. This awareness of a concrete whole is similar structurally to the awareness and physical object as a ‘bundle of sensations’—a philosophical move common to much British empiricism. One might argue that at this level, the empiricism connected with immediate sensation as found in Aquinas converges with the later British cohort of philosophical empiricists. However, the internal sense faculties of the vis cogitativa and the sense memory provide a radical bifurcation between Aquinas and the British empiricists. In effect, the sensus communis is that part of the external sensorium, which accounts for the unity of sense awareness. This unity comes about because there is a single faculty, which is the root or source of the five external senses. The unity does not come about because the sensus communis forms a new ‘object’, be it an image, phantasm, or sense datum, from the data of the five external senses. Aquinas’s rejection of psychological atomism is rooted in the unity of the external sensorium—a unity which is explained by means of the sensus communis.
The next part of this analysis begins the discussion of the three faculties of inner sense that require as a necessary condition for their functioning the existence of a phantasm. These are the imagination, the vis cogitativa, and the sense memory. Appendix 1 is a brief historical excursus on the role played by the ventricle system in medieval discussions of the physiological organs of the internal sense faculties
-  It is certainly unclear what role the degree of ‘moistness’ plays in all of this! Nonetheless, this accountappears to be part of the general physiological wisdom of the day, as Thomas uses this description severaltimes in his writings.
-  Kenny, Aquinas on Mind, 31. Kenny refers to Summa Contra Gentiles, bk II, no. 73.