The Intentional Awareness in Sensation
Given the three necessary conditions without which sensation cannot take place, what is required next is a consideration of the act of sensation itself utilizing these three necessary conditions. Lacking any one of these necessary ingredients entails no mental act of awareness. It is because of the necessary character of all three terms that Aquinas asserts claims like the following: ‘Consequently, the judgement of the sense about proper sensibles is always true unless there is an impediment in the organ or in the medium (De Veritate I, 11; emphasis added). Aquinas places all three categories—object, medium, and organ/faculty—on the same plane. Hence, it is consistent with the texts to propose that Aquinas opts for a necessary triadic relation as the set of necessary conditions for the act of sensation using the external senses. This triadic relation is best elucidated under the title of ‘objective relativism’. Objective relativism suggests that the context of the awareness depends on the special exemplification of each necessary condition. To alter any condition produces a different content for the act of awareness. The awareness is ‘objective’ because the causal structure of the proper or common sensible is isomorphic with the esse intentionale in the sense faculty. The awareness itself, however, depends upon an exemplification of the other two necessary conditions.
Aquinas suggests that if one of these terms of the triadic relation is deficient in any way, the possibility for veridical perception diminishes in that particular instance. For example, if the eye pupil is a colour-blind organ, then no matter how luminous the medium might be nor how pristine the colour as a causal factor, the perceiver will not see the object as properly coloured, because colour-blindness affects perceiving the particular colour in question. In a similar vein, he argues that the tongue of a person affected with the flu will not permit her to taste sweet and sour, which would be the case were the ‘taster’ well and with normal perceptual conditions holding. The following passages from the Commentary, the Summa Theologiae, and the De Veritate stress the importance of a properly disposed faculty and organ in order for veridical perception to occur:
For, in the first place, sense perception is always truthful with respect to its proper object, or at least it incurs, with respect to these, the minimum of falsehood; for natural powers do not, as a general rule, fail in the activities proper to them; and if they do fail, this is due to some derangement or other. Thus only in a minority of cases do these senses judge inaccurately of their proper objects, and then only through some organic defect, e.g. when people sick with fever taste sweet things as bitter because their tongues are ill-disposed. (Commentary on the Soul, no. 661)
A second text: ‘We say that the senses are not deceived regarding their proper sense object, except by interference and in abnormal cases and when the sense organ is impaired’ (Summa Theologiae, I q. 17 a. 2). In Aquinas’s earlier Quaestiones Disputatae de Veritate,3 we find the following account, which suggests that Aquinas remained fairly consistent in his analysis of sense knowledge:
The judgement of sense about certain things—for example, proper sensibles—takes place naturally—i.e. spontaneously. About other things, however, it takes place by means of a certain comparison, made in the human perceiver by the cogitative power [vis cogitativa], a sense power, whose place in animals is taken by a spontaneous estimation [vis aestimativa]. Thus, the powers of sensation [i.e. the external senses and the vis cogitativa] judge about common sensibles and the incidental object of sense—i.e. the accidental sensibles. However, the natural—i.e. spontaneous—action of a thing always takes place in one way, unless by accident it is impeded intrinsically by some defect or extrinsically by some impediment. Consequently, the judgement of sense about proper sensibles is always true [i.e. veridical] unless there is an impediment in the organ or in the medium; but its judgment about common sensibles or the incidental objects of sense—i.e. the accidental sensibles—is sometimes wrong. Thus, it is clear how there can be falsity in the judgment of sense. (De Veritate II, q. 1 a. 11)
Even at an early date, Aquinas wrote about the vis cogitativa and the incidental object of sense, themes that remained central to this philosophy of mind over his lifetime.
There is a relation of dependency of the faculty to the organ. Aquinas claims that a defective organ contributes to a faculty’s sensing incorrectly or inappropriately. ‘Appropriate’ and ‘inappropriate’ probably are the useful terms here. It seems to be the case that a defective organ contributes to a defective faculty because the faculty cannot function in the appropriate fashion. This indicates again the naturalistic epistemology upon which Aquinas bases his theory of cognition. A properly working organ is a necessary condition for a properly functioning sense faculty. 
The previous analysis in terms of Disposition-2/Actuality-1 offers an explanation for the above passages. Aquinas claims that the sense organ itself must be properly disposed if it is to contribute to sense perception. If an adequate disposition is absent, veridical perception has no possibility of taking place. The ‘ratio’ of the organ must be such that it is capable of an act of sensation. In addition, it is necessary that the medium be appropriate. The earlier discussion of light as essential for the visual sensation of a proper sensible renders this claim obvious. Thus, no matter how perfect the eye—i.e. it is properly disposed to perceive all colours—and how potentially visible the colour, if there is not an adequate medium—i.e. if the intensity of light in the medium is not sufficient for the colour to react with it—then again there is no possibility of sensation. These considerations justify proposing that Aquinas’s position on sensation can best be explained through a necessary triadic relation, NC[O-M-F]. The sufficient condition needed to make the necessary conditions function is the working of the sensus communis, which is the ‘root’ (radix) of consciousness. The necessary conditions by themselves are not sufficient to provide an act of awareness. This point will be discussed in the next chapter.
In his Quaestiones Disputatae de Veritate, Thomas writes more about the structure of awareness; all three necessary components of the necessary triadic relation are discussed in this passage:
In a given act of awareness of seeing, a threefold division can be discerned. One is the medium under which it is seen. The second is that by which it is seen, and this is the intentional likeness [similitudo] of the thing seen. The third is that from which one obtains sense knowledge of the thing seen. Thus, for instance, in bodily sight, the medium under which a thing is seen is light; by which the thing becomes actually visible and the power of sight is given the perfection of seeing. The means by which a thing is seen is the sensible species itself of the thing existing in the eye, which, as the form of the one seeing in so far as she is seeing, is the principle of the activity of sight. And the means from which one receives knowledge of a thing seen is like a mirror, from which the eye at times receives the species of some visible thing, for example, a stone, and from the stone itself. (Quaestiones Disputatae de Veritate, II q. 18, a 1, replies to objections; emphasis added)
Aquinas is aware, to be sure, that instances of non-veridical sensation happen; these issues were covered earlier in this chapter.
-  Torrell dates this treatise from 1256-9, which would be the first period that Aquinas taught as a Masterin Paris: Saint Thomas Aquinas, vol. 1: The Person and His Work, trans. Robert Royal (Washington, DC:Catholic University of America Press, 1996), 334.