The Three Categories of Similitudo
In discussing the mental acts both of sensation with the external sensorium and of phantasm formation with the internal sensorium, Aquinas frequently uses ‘similitudo’. As noted above, this term is used in one definition of phantasm: ‘dicendum quod etiam ipsum phantasma est similitudo rei particularis’ (Summa Theologiae, I q. 84 a. 2). In order to be understood successfully, the concept of similitudo, like the concept of phantasm itself, requires careful elucidation. In her Aquinas, Stump is one of the few commentators on Aquinas’s philosophy of mind who expresses any theoretical concern about the concept of similitudo: ‘The Latin “similitudo” is commonly translated as “likeness,” and this translation has given some readers the impression that a similitude pictorially resembles the thing of which it is a similitude.’ She goes on to suggest: ‘But this is at best a very misleading impression.’ She offers the significant insight that in some way a similitudo is ‘an agreement in or sharing of forms’. Aquinas writes: ‘There is a similitudo between two things insofar as there is agreement in form’ (De Veritate, q. 8 a. 8). In the Prima Pars of his Summa Theologiae, Aquinas devotes an entire article in question 3 on the concept of similitudo and how creatures are like God. This entire article will not be considered here, but its opening sentence is important for this discussion. ‘Since similitudo is based upon agreement or communication in form, similitudo varies according to the many modes of communication in form’ (Summa Theologiae, Ia q. 4 a. 3). Stump’s development of this theme is important in understanding the nature of a similitudo; but she does not go far enough in her analysis of this important concept.
In addition to the above definition, the following texts denote a reference to ‘likeness’ or ‘similitudo’. ‘However, phantasms are likenesses of individuals and exist in bodily organs of sense’ (Summa Theologiae, I q. 85 a. 1 ad 3). ‘Thus, material things must be understood insofar as they are abstracted from material likenesses, that is, phantasms’ (I q. 85 a. 1). In these passages, the phantasm is used as a kind of likeness or resemblance originating from an individual, material object in the external world. Insofar as this would be an image, it would refer partially to Position-B. The likeness is a composite resemblance of the individual thing, which has been sensed. In effect, structurally this use of likeness as a phantasm would be a ‘residue’ of the unified composite whole sensed by the sensus communis from the discrete data received from the external senses. The precise analysis of ‘residue’ will occur later where it refers to a ‘retaining’ of the content of a direct awareness by the external sensorium. This residue, however, is not an object of direct awareness by means of the external sensorium.
This use of likeness as phantasm, however, is not the only use of the term found in the writings of Aquinas. The following passages indicate a new twist given to the very same term: ‘Hence that by which the sight sees is the likeness of the visible thing. [ . . . ] The likeness of a sensible thing is the form of the sense in act’ (Summa Theologiae, I q. 85 a. 2). ‘However, the sensible species or likeness is not what is perceived, but rather that by which the sense perceives’ (I q. 85 a. 2, sed contra; emphasis added).
In these passages, Aquinas uses likeness, or sensible species, in reference to direct sensation. It is correct that a sensible species is the immaterial likeness of an immediate datum of sensation; accordingly, a red or a square, or any other of the proper or common sensibles, is a likeness obtained in direct sensation. If the previous discussion of the phantasm is correct—i.e. a phantasm is never found during the functioning of the external sensorium alone—then this likeness which occurs during direct perception must be different from the likeness that Aquinas used in considering phantasms. A phantasm, it has been established, is never utilized during direct sensation. Accordingly, there must be at least two different senses of likeness at work in Aquinas’s theory of sensation and perception.
There is, however, one more example of likeness/similitudo, which should be considered albeit briefly. Aquinas also uses likeness in discussing concept formation. However, since he employs the term similitudo in that discussion, it is appropriate to consider this additional function for similitudo in Aquinas’s theory of knowledge. The following texts illustrate Aquinas’s use of likeness in considering the process of ‘understanding’: ‘The likeness through which we understand is the species [likeness] of the thing known in the knower’ (Summa Theologiae, I q. 85 a. 8 ad 3); the Latin text is very clear on this use of similitudo: ‘conceptio intellectus est similitudo rei intellectus’ (I q. 27 a. 2). In this discussion two different philosophical categories are being treated: the intelligible species and the concept, which later scholastics refer to as the species expressa—sometimes referred to as the ‘verbum mentis’—in the intellectus possibilis. In this respect, Aquinas suggests that the intellectual species is that by means of which a knower is able to have intellectual or conceptual knowledge. To use medieval terminology, this use of likeness is the means by which a knower is aware of essences or attains to a knowledge of a quidditas. It is the ‘a quo’ through which intellectual awareness takes place. John of St Thomas refers to this as an ‘intellectual’ species impressa. It is a means by which a person knows a concept, not the intentional object of the concept itself.
In light of the above textual evidence, it appears that Aquinas has three distinct and different uses of similitudo. This set can be delineated in the following way: Likeness-1, Likeness-2, and Likeness-3. Textually, Aquinas himself appears not to have distinguished these various uses. John of St Thomas (Poinsot) introduced the terms ‘species impressa’ and ‘species expressa’.50 Although helpful, these distinctions of John of St Thomas, which have been adopted generally by latter-day scholastic philosophers, alone do not offer a complete explanatory account of likeness. This point should become clear as this discussion unfolds.
Likeness-1 is the actual disposition of the sense faculty following from the efficient and formal causal influence of the sensible quality as an active causal power on the sense organ, which is received from the sensible object in the external world. This disposition makes the sense faculty of sight, for example, ready and able to perceive ‘red’ rather than ‘purple’. This uses of Likeness-1 entails that it is a means or necessary condition for sensation.51 Likeness-1, however, is not the object of sensation. Therefore, Likeness-1 is not an imago as used in Cartesian epistemology, nor a sense datum as used in early twentieth-century analytic philosophy.
Likeness-2 is the ‘remnant’ or ‘residue’ of the actual sensation that occurs by means of the external sensorium. It will be analysed in terms of the necessary conditions of perception for the external sensorium. Generally speaking, Likeness-2 is what one might common-sensibly refer to as ‘experience’, as when one distinguishes the seasoned veteran from the rookie in that the former has more ‘know-how’ than the latter. This sense of likeness is an important aspect for what the next chapter suggests is the correct analysis of phantasm. Likeness-2, when phantasms are considered directly, will be sub-divided further into Phantasm-1, Phantasm-2, and Phantasm-3.
Likeness-3 is the intellectual species, which is the ability or means to possess intellectual knowledge and thus form concepts in the intellectus possibilis as expressed in Aquinas’s philosophy of mind by having a ‘conceptus’. In the Commentary on the Soul, Likeness-3 is what Aquinas refers to as an acquired intellectual disposition. One probably needs to make a further distinction regarding Likeness-3. Likeness-3a is the determination of the intellectus agens through the process of abstraction by means of which the species intelligibilis is made. Likeness-3b is the similitudo that is the conceptus in the intellectus possibilis, which is the means by which one understands the essence existing in the primary substance of a natural kind. Furthermore, Likeness-3b is what Geach, in Mental Acts, referred to as a ‘capacity’ or ‘disposition’.52 A dispositional property is Geach’s analysis of an acquired concept. To put the matter differently, this ‘acquired   
intellectual disposition’ or Likeness-3b is what Geach understood to be Aquinas’s analysis of ‘having a concept’; this would be an example of Disposition-II/Act-I. Likeness-3b is that by means of which humans know the essence or natural kind or the set of sortal properties of an individual primary substance.
One further comment must be made concerning Likeness-2. To illustrate this point, one must consider again the following passage from the Summa Theologiae:
In a human person’s sensitive knowledge, there are two operations. One is limited to immuta- tion. This operation of the senses occurs when the sensible object in the external world impresses the senses. The other operation is formation. The imagination, for example, forms for itself an image of an object, which is absent, or even of an object, which was never seen before. (Summa Theologiae, I q. 85 a. 2 ad 3; emphasis added)
The first operation, what Aquinas refers to as ‘immutation’, is Likeness-1. This is the likeness formed in the sense faculty by the causally efficacious sensible object outside of the mind. This likeness is what renders the faculty disposed for sensation of a specific proper or common sensible. The second operation, what Aquinas refers to as ‘formation’ in the above passage, forms an image (imago or indolum) or species expressa: e.g. ‘Pegasus’ or ‘golden mountain’. This analysis argues that this formed image, however, is not per se the phantasm either. This image is a sufficient condition for the existence of a phantasm but it is not a necessary condition. It is interesting to note that the text considered here uses the Latin term idolum, and not similitudo. Accordingly, idolum is the result of the creative capacity of the vis imaginativa.
The next chapter, when considering the logic of phantasm as a Likeness-2, will note four aspects that are included under the concept of ‘phantasm’:
- (a) the idolum itself;
- (b) the sensible material content—i.e. the residue—from sensation from which the vis imaginativa creatively forms an idolum;
- (c) a structured awareness of an individual by means of the vis cogitativa;
- (d) the sensible material content stored in the vis memorativa from the vis cog- itativa and that from which the intellectus agens abstracts or ‘makes’ a species intelligibilis.
Structurally, (a) and (d) above are quite similar. Their difference consists in different inner knowing faculties operating on a ‘residue’. In the first case, the vis imaginativa is active on the residue from the sensus communis and stored in the imaginatio; in the second case, the intellectus agens is active on the residue from the vis cogitativa and stored in the vis memorativa. The next chapter distinguishes the different uses of phantasm in some detail. In effect, however, any view that reduces a phantasm merely to an image alone is not an adequate structural analysis of Aquinas’s theory of mind. Likewise, any reductive or eliminative attempt to render phantasms into sense data is inconsistent textually and structurally with Aquinas’s philosophy of mind.
-  Stump, Aquinas, 255.
-  ‘Sensible species’ is being used here, referring to the terminology of John of St. Thomas (Poinsot),to mean a ‘species impressa. The sense organ and the sense faculty distinction is important in directsensation.
-  A detailed analysis of concept formation is beyond the bounds of this present inquiry.
-  Although the terms species impressa and species expressa are commonly attributed to the writings ofAquinas, it is difficult to discover these exact terms in any text of Aquinas. However, they are found oftenin the writings of John of St Thomas. Cf. Cursus Philosophicus Thomisticus ‘Philosophia Naturalis’, IV q. 11a. 1, p. 362.
-  In the analysis of sensation via the external sensorium considered earlier, a necessary triadic relationwas introduced for sensation; in schematic notation [NC (O-M-F)] refers to the necessary conditions ofevery mental act of sensation by the external sensorium. These conditions, it will be recalled, are: (a) anobject in the external world, (b) an adequate medium, and (c) a properly disposed sense organ and faculty.The sense faculty is dependent on the disposed sense organ. ‘Likeness-1’ refers to the ‘F’ in the schematicnotation. Likeness-1 is the disposed faculty dependent on the sense organ, which is ‘able to perceive’ whenjoined with the other two necessary terms of the triadic relation. It must be emphasized that Likeness-1 isnever a tertium quid.
-  Peter Geach, Mental Acts (London: Routledge & Kegan Paul, 1957), 11-18, 33-8.
-  I am much indebted to James South for indicating a muddle on ‘intellectual species’ and ‘concept’ inan earlier draft of this chapter.