The functioning of the external sensorium is a necessary but not a sufficient condition for the intentional existence of each type of phantasm. An earlier chapter suggested that the most perspicuous way to characterize Aquinas’s theory of sensation is by means of a three-term necessary relation. This necessary triadic relation has for its terms the following: (a) the disposed sense organ and faculty; (b) an adequate medium; and (c) a proper or common sensible existing as an active causal factor in the external world.
In order for perception to occur within the external sensorium, a referent for each of these terms is necessary. For example, in order for sensation to occur with the mental act of seeing, the sight perceiver must meet the following three conditions: (a) The perceiver must have a sense organ and a sense faculty capable of seeing colour; (b) there must be a sufficient intensity of light; and (c) there must be a coloured object existing as a causal factor in the external world.
This account renders Aquinas an objective relativist in his theory of sensation.  This would be in agreement with Stump’s claim that Aquinas is best categorized as an externalist with a reliabilist thrust. The combination of these factors, which is the triadic relation, is a necessary condition for the functioning of the external sensorium but not a sufficient condition. This is the case because the sensus communis is also a necessary condition. Therefore, this necessary triadic relation together with the functioning of the sensus communis establish a sufficient condition for acts of awareness to occur in the external sensorium. In other words, the conjunction of the necessary triadic relations together with the functioning of the sensus communis is the sufficient condition for sensation with the external sensorium. Schematically, this necessary triadic relation might be expressed as follows: [NC (O-M-F)]. Quite perspicuously, this refers to a necessary triadic relation encompassing as terms the object as a causal factor in the external world, an adequate medium, and a properly disposed organ and faculty
This descriptive account of the workings of the external sensorium is necessary in order to understand the following elucidation of the structure for Phantasm-1; the vis imaginativa is referred to by Aquinas as a ‘storehouse’ of the sensible forms.
for the reception of sensible forms, the proper sense and the sensus communis [i.e. the external sensorium] are necessary. [. . .] But for the retention and preservation of the forms perceived by the external sensorium, the phantasia or imagination [vis imaginativa] is necessary. The imagination is, as it were, a storehouse [thesaurus] for the forms received through the external sensorium. (Summa Theologiae, I q. 78 a. 4)
This passage implies that there is a ‘residue’ or ‘remnant’ left from each mental awareness of the external sensorium. Phantasm-1 is this residue or remnant of what was perceived in accord with the necessary conditions of sensation using the external sensorium. The intentional residue itself is not necessarily an object of awareness. It is merely the intentional remnant of an actual mental act of awareness by means of the external sensorium. It is possible, however, for Phantasm-1 to become an object of knowledge. This will be the case when Phantasm-1 functions as an idolum or imago or what the latter scholastics called a species expressa. Yet a Phantasm-1 need not have this epistemological status. It is sufficient that it just serve as an ‘unconscious’ remnant or imprint of a previous mental act of the external sensorium. When this type of imprint of a prior awareness itself becomes the object of an additional act of awareness, then Phantasm-1 functions as an imago. This point will be discussed later.
Phantasm-1, however, must not be confused with Likeness-1. Likeness-1 is one of the necessary conditions for sensation; to be more specific, it is the properly disposed sense organ and faculty. Sensation itself by means of the external sensorium, on the other hand, demands all three necessary conditions (the properly disposed organ and faculty, an adequate medium, and an appropriate active causal object or power in the external world) together with the mental act of the sensus communis. What is stored in the imagination, therefore, is a residue or remnant of the content sensed through the external sensorium. Each awareness of a proper or common sensible is attained precisely insofar as it is an instantiation of this necessary triadic relation. The content as sensed through the sensus communis is a conjunction of sensations determined by a series of necessary triadic relations. The product of this conjunction, which is the way a bundle of qualities is perceived by the external sensorium, is what, as an intentional residue, becomes Phantasm-1. The structural similarity between the conjunction as perceived through the sensus communis and the ‘bundle of sensations’ spoken of by Berkeley and Hume is obvious. But whereas both Berkeley and Hume stop their analyses of perception with the awareness of a ‘bundle of sensations, Aquinas—by means of a structured awareness using another aspect of phantasms, Phantasm-2—greatly refines this account of sense perception. This last point will be developed later in this chapter with the analysis of what will be called Phantasm-2.
Phantasm-1, therefore, functions as an imprint in the vis imaginativa whose content is the product of the conjunction of a series of sensations determined by a necessary triadic relation for each sense experience. In other words, the sensible species or intentional form in the sensus communis is ‘implanted’, as it were, in the internal sense dispositions of the vis imaginativa. This implanted sensible species becomes an acquired cognitive disposition, which is an instance of Disposition-II/ Act I. This product is what is implanted in the vis imaginativa. This interpretation is in accord with Aquinas’s textual claim that the imagination is a ‘thesaurus’ for the sensible forms received from the external sensorium. Accordingly, Phantasm-1 is a token of a type of Likeness-2.
Upon examining the philosophical narratives concerning the role phantasms play in Aquinas’s theory of sensation and perception, most commentators, both within the traditional scholastic school and philosophers from the analytic tradition, have dwelt primarily on this one aspect of the phantasm. There is more to a complete elucidation of the nature of phantasms, however, than this one function. In addition, some students of scholasticism have written as if the species impressa is equated with the phantasm. This, however, cannot be the case. The species impressa is both identical and coextensive with Likeness-1. Likeness-1, in turn, is only one of the necessary conditions of a three-term necessary relation for external sensation. It follows that the phantasm and the species impressa are categorically different and distinct epistemological categories. Secondly, the phantasm is neither a sense datum nor necessarily an intentional image. Some commentators have implied that the phantasm is to be understood at all times as an image. This is Position-B discussed in the preceding chapter. Neither of these positions is acceptable in toto because Phantasm-1 is to be understood as the residue or remnant of a sensation implanted in the cognitive dispositional power of the imagination. This imprint is beyond actual sensation or direct awareness by the external sensorium. A Phantasm-1 does not need to be an object of awareness. It is merely a residue of a prior act of direct sensation.
Before considering Phantasm-2, one further aspect of the vis imaginativa requires clarification. Aquinas provides the vis imaginativa with three functions:
With this third intentional function, one can form, for example, the image of Hume’s famous ‘golden mountain’. ‘Human perceivers have knowing powers that, from likenesses first perceived, can form others—as when we use the imagination to form an image of a golden mountain from those of gold and a mountain’ (Summa Theologiae, I q. 12 a. 9 ad 2). 
The second and third functions are also uses of a phantasm. In accordance with the discussion in the last chapter, however, it is more properly called an idolum or an imago? Aquinas is somewhat blurry on this matter of idolum or species versus phantasm. In the Commentary on the Soul, he writes: ‘Phantasms come during sleep when the senses are not in act’ (Commentary on the Soul, no. 647). Aristotle renders this distinction: ‘Imagination is the movement engendered by sensation in act, and the dream appears to be a certain phantasm, since we call the dream the phantasm in sleep’ (De Anima, 459a170). Accordingly, a dream image as well as a formed image of something never seen belong to the mental acts of the imagination. A phantasm may also be a remembered image of an earlier sensation. If these are also instances of a phantasm, then they must be distinguished from Phantasm-1. One might call these uses Phantasm-la and Phantasm-lb. However, it might be preferable to classify the remembered image, the dream image, and the ‘created image’—daydreaming, as it were—as an idolum, and thus not introduce a further analytic distinction into this conceptual analysis of phantasms. Suffice it to say that a Phantasm-1 need not always be the object of an act of direct awareness by the imagination. However, when it is an object of direct awareness, either in remembering, dreaming, or imagining, then the phantasm functions as an idolum or an imago. An idolum is what John of St Thomas referred to as a species expressa.
Yet Phantasm-1 can occur without being an object of direct awareness. It can just be ‘there, as it were. Phantasm-1 utilizes the thesaurus function of retaining the residue in the imagination from a prior direct perception by the external sensorium. The imagination itself is the cognitive potency; Phantasm-1 is there as a residue which can become an object of awareness should the imagination direct attention to this residue, e.g. an idolum. When a Phantasm-1 functions as an idolum, a new act of awareness is required for which the idolum is its intentional object. But a Phantasm-1 need not have this second act of awareness in order to have epistemological status in Aquinas’s philosophy of mind. It is a sufficient condition that a Phantasm-1 is an image, but it is not a necessary condition. A Phantasm-1 satisfies the epistemological demands for its intentional existence merely by having esse intentionale as a residue without being the object of an additional act of awareness. Therefore, an additional act of awareness is necessary for a Phantasm-1 to function as an idolum; Phantasm-1 can function as a residue without this act of awareness. Not every Phantasm-1 is a species expressa and it never is a species impressa. However, a Phantasm-1 becomes an idolum, imago, or species expressa if it becomes the intentional object of a second mental act of direct awareness of the vis imaginativa. This might be (as noted above) called a Phantasm-1a. It is important to realize that it is only in this idolum or species expressa function that a Phantasm-1 serves as an object of an act of direct awareness. If the image interpretation of a phantasm were correct, then the phantasm would always be a direct object of knowledge. Phantasm-1 is only a direct object of knowledge when it functions as an idolum or species expressa, i.e. as an object of an act of awareness by the vis imaginativa. Therefore, Phantasm-1 cannot be used in support ofthe claim that Aquinas is a representa- tionalist in perception. Furthermore, neither can it be used to support the claim that the phantasm is always an image. Therefore, Phantasm-1 cannot be used for substantiating either Position-A or Position-B when considering the status of image discussed in the preceding chapter. That a phantasm is never involved in a direct act of perception by means of the external sensorium refutes Position-A. That a phantasm can function as a mere residue and not as an idolum refutes a strict interpretation of Position-B. Yet Position-B is partially correct in that a phantasm does have the idolum function. Nonetheless, the idolum function is not a necessary condition for a phantasm’s existence. The consideration of Phantasm-2 next will provide additional support for the claim against a strict interpretation of Position-B.