The Four Internal Senses
These texts reveal that for Aquinas, a simple direct awareness of sensibles, while necessary, is not sufficient to account for sensation and perception. There appears to be a fourfold reason for his claim, each of which indicates the need for a unique faculty of the internal senses.
(a) First, Aquinas claims that knowers not only are directly aware of brute sensations, which are the proper and the common sensibles, but they are also simply aware of complete ‘wholes’. A perceiver is not aware only of a red colour patch, but of the red patch together with a certain shape, a certain size, and so forth with the rest of the proper and the common sensibles pertaining to any given perception. The awareness of ‘complex wholes’ as well as a discrimination of the different genera of the proper sensibles, one from another, force Aquinas into postulating the faculty of the sensus communis.
The proper sense [external sensorium] judges of the proper sensible by discerning it from other things, which come under the same sense. For instance, by discerning white from black or green. But neither sight nor taste can discern white from sweet, because what discerns between two sense qualities must know both. Hence, this discerning judgement must be assigned to the sensus communis. To it, as to a common term, all apprehensions of the senses must be referred, and by it, again, all the intentions of the senses are perceived. (Summa Theologiae, I q. 78 a. 4 ad 2)
This unifying function of the sensibles comes about because the sensus communis is the common root of the external sensorium. A text from the Commentary makes this point: ‘Now the sensitivity flows to the organs of all the five senses from one common root, to which in turn are transmitted and in which are terminated, all the sensations occurring in each particular organ’ (Commentary on the Soul, no. 609). The sensus communis, therefore, synthesizes a composite sensible, or a concrete whole, from the discrete data received from the external senses.
- (b) Secondly, there is a need for an additional sense faculty, which Aquinas refers to as the phantasia or imagination. This faculty retains or conserves the complex impression which has been perceived as a unified whole by the sensus communis. ‘For the reception of sensible forms, the proper sense and the sensus communis are appointed. [. . .] But for the retention and preservation of these forms, the phantasia or imagination is appointed, being as it were a storehouse [thesaurus] of forms received through the senses’ (Summa Theologiae, I q. 78 a. 4; emphasis added). In the Summa Contra Gentiles, Aquinas writes: ‘The powers, which preserve the forms that are not actually being apprehended, are not “apprehensive” powers, but “storehouses” [thesauri], of the apprehensive faculties; for example, the imagination, which is the storehouse of forms apprehended by the sense’ (Summa Contra Gentiles, bk II, ch. 74). The second of the internal senses, therefore, is that faculty which conserves or retains sense impressions gained through the external sensorium. In the Summa Contra Gentiles, the Latin term used by Aquinas for the imagination is ‘imagination
- (c) Aquinas’s third reason for positing the internal sensorium is his conviction that human perceivers are aware of individuals as specific entities and not just as the ‘bundles of sensations’ so common to British empiricism. The exact nature of the vis aestimativa in animals and the vis cogitativa in humans will be analysed later. Suffice it to say now that the mental act of the former apprehends that which is agreeable or disagreeable or that which is useful or to be feared, while the latter perceives the 
individual as a concretum. Aquinas argues that this facet of sense experience cannot be explained through direct awareness when referring to the external sensorium alone. Accordingly, he needs another internal sense, which he refers to as the vis aestimativa in animals having only sense knowledge and as the vis cogitativa in human perceivers. Aquinas discusses these faculties in the Summa Theologiae, the Summa Contra Gentiles, and the Commentary on the Soul, among other places:
Furthermore, for the apprehension of intentions that are not received through the senses, the aestimative power is appointed. [. . .] Therefore, the power, which in other animals is called the natural aestimative [vis aestimativa], in human persons is called the cogitative [vis cogitativa], which by some sort of comparison discovers these intentions. (Summa Theologiae, I q. 78 a. 4; emphasis added)
Accordingly, it [vis cogitativa] is aware of this human person as this individual human person and this tree as this tree. (Commentary on the Soul, no. 398)
(d) Fourthly and lastly, a correlative principle to the vis aestimativa in animals and the vis cogitativa in humans is not only that are these are ‘unsensed intentions’—what later scholastic philosophers would refer to as ‘intentiones non sensatae, an object of awareness—but that these perceptions also are conserved and retained. This ability to conserve these ‘intentiones non sensatae’ is the function of the sense memory: ‘And for their preservation [the intentiones not received through the external senses], the memorative power, which is a storehouse for such intentions’ (Summa Theologiae, I q. 78 a. 4; emphasis added). Aquinas also writes in the Summa Contra Gentiles: ‘The memory, which is a second storehouse of intentions, this one for intentions apprehended without the senses, as when the sheep apprehends the enmity of the wolf’ (Summa Contra Gentiles, bk II, ch. 74; emphasis added).
The method used in establishing the faculties of the internal senses is similar to what Aquinas used with his explanatory account of the external senses.
- (a) A human perceiver is aware of complex wholes.
- (b) A human perceiver retains the awareness of these wholes.
- (c) A human perceiver is aware of individuals and not just of mere bundles of sensations.
- (d) A human perceiver retains that awareness of individuals as individuals.
In order to account for these four distinct internal sense mental acts, Aquinas posits the four internal senses of sensus communis, imagination (vis imaginativa), vis cogitativa (the vis aestimativa in animals), and sense memory (vis memorativa). In other words, Aquinas is convinced that these four internal sense faculties are necessary in order to account for the pre-analytic facets of sense knowledge and perceptual awareness. This is a continuation of his faculty-based philosophy of mind. He uses this mode of explanation as opposed to epistemic justification throughout his philosophy of mind.
-  The Latin text for this passage is the following: Virtus autem quae conservant formas non apparehensasin actu dicit non esse vires apprehensivas, sed thesaurus virtutum apprehensivarum; sicut imaginatio, quae estthesaurus formarum apprehensarum per sensum: Summa Contra Gentiles (Rome: Marietti, 1946).
-  The Latin text of the preceding passage from the Summa Contra Gentiles reads as follows: ‘Memoria,secundum ipsum, quae est secundus thesaurus intentionum apprehensarum abseque sensu; sicut quum ovisapprenhendit inimieitiam lupi (ibid.).