Aquinas’s Modification of ‘Like Knows Like’
In beginning his extended analysis, Aquinas considers the second proposition, rooted historically in Empedocles but further developed in the Platonic tradition. He suggests that this proposition, ‘Like knows like’, is central to any analysis of sensation and perception. However, this principle must be interpreted in a radically different manner from what Empedocles claimed. In this reinterpretation of ‘Like knows like, Aquinas begins his analysis of intentionality.
Historically, Empedocles argued that perception occurred because an identity holds between objects within and without the knower. This isomorphism, moreover, is one of strict formal identity. In the following brief passage taken from one of the few surviving Empedoclean fragments, Empedocles discusses sight: ‘For it is with earth that we see Earth, and Water with water; by air we see bright Air, by fire destroying Fire. By love do we see Love, and Hate by grievous hate’ (Fragment 109). Empedocles explained the possibility of knowledge through the actual presence in the knower of an element identical to an element outside the mind. This is a mechanistic model, which denies any significance to intentionality as an ontologically primitive characteristic of knowers. 
Aquinas modifies Empedocles’s axiom. In effect, his modification attempts to rule out the ‘frankly unbelievable’ status Smart attributed to sensations and perceptions. Subsequent to Aristotle’s De Anima, this modification contributed to the development of intentionality theory. Aquinas utilizes the act/potency distinction in this modification of Empedocles’s principle. He claims that the sensitive soul, which in effect grounds a perceptual disposition, contains its object only potentially and not actually. Aquinas understands the rudimentary Empedoclean epistemology literally. He suggests that Empedocles asserted that the ‘actual existence’ of the elements within the knower accounted for the possibility of knowing those elements outside of the mind. Aquinas evaluates critically this interpretation of ‘Like knows like’ in terms of an ‘immaterial’ reception of forms. In responding to Empedocles, in effect, Aquinas develops his own thesis of intentionality. If the sense faculties actually and materially contained their objects, then, Aquinas suggests, any thesis of intentionality would be undercut. He provides two reasons for this consequence, both serving as counterexamples: (a) any knower would be able to sense her own sense faculties by means of the faculty under consideration; (b) sensations could be had even though the objects themselves were not present.
Proposition (b) is related directly to the general criticisms concerning representative realism. Descartes articulated the philosophical problem that if representationalism were correct, then a human knower might be locked in his own mind, which would entail that ideas would never connect with the external world. Malebranche developed this same set of epistemological problems, denying the basic ‘tending towards’ and ‘aboutness’ relation characteristic of the ontological state of intentionality.
Aquinas too suggests that if Empedocles’s theory is true, it follows that not only is it a theory of representationalism, but it also suffers a principal defect of representation- alism. Aquinas indicates that if a strict interpretation of the Empedoclean axiom is accepted, then the objects of sensation would be contained materially—i.e. existentially ‘fleshed out’—within the structure of the sense faculties themselves. If this were correct, then it follows, so Aquinas suggests, that a perceiver could perceive her very sense faculties. In this analysis, Aquinas implicitly utilizes the act/object distinction. He does not deny that one can be aware of one’s own acts of awareness. In other words, Aquinas does not deny the possibility that knowing beings, as beings capable of acts of intentionality, have the ability, even at the perceptual level, of reflective self-awareness. In fact, he suggests that the sensus communis is the faculty by which perceptual selfawareness occurs. Accordingly, in the above argument against a literal reading of the Empedoclean axiom, Aquinas must be implying that the ‘sense faculty’ itself and not the ‘act of awareness’ would become an object of perception. In other words, he is not considering the mental act of the sense faculty. On the experiential level, Aquinas does have a point. It is impossible to see one’s eyeball or hear one’s eardrum, and so forth. Aquinas’s concern, then, is that if Empedocles’s axiom is viable, then given a theory of perception following from this axiom, several odd conclusions follow.
The second consequence of a literal reading of the Empedoclean axiom is a counterexample in favour of direct realism. In one sense, Aquinas simply restates what he means by perception—i.e. an awareness of an object in the external world. His discussion once again is characteristic of the strongly realistic overtone found throughout his theory of sensation and perception. In a strict sense, one might claim that Aquinas begs the question against Empedocles. On the other hand, Aquinas sees no compelling reason or set of reasons for accepting representationalism. Put simply, if a theory of perception either denies an awareness of the things found in the external world or contains a structural component entailing such a denial, Aquinas considers this fact alone sufficient for a reductio ad absurdum argument against the theory’s tenability.
Here he is strikingly similar to some mid-twentieth-century ordinary-language philosophers, Ryle and Austin in particular. Of course, it does not follow that Aquinas was an ordinary-language philosopher. Nonetheless, the common-sense dimensions to several aporia articulated by Aquinas have similarities to issues central to the philosophy of mind in mid-twentieth-century analytic philosophy. These similarities run through much of Aquinas’s writings.
In using the concept of ‘common-sense knowledge’ in this context, Aquinas suggests that his tenets regarding the philosophy of mind underwrite common-sense realism; yet common sense is also a means of explicating the appropriateness of the correct application of concepts as well as the assertion of true propositions that belong to an Aristotelian account of scientific knowledge. In other words, Aquinas’s notion of the philosophy of mind is not limited to an unreflective common-sense realism about there being a public world of objects—although this certainly plays a role. The philosophy of mind is the basis for common-sense knowledge, suggesting that there is a world of primary substances of a natural kind. From this beginning, Aquinas develops his philosophy of mind as a way of explaining how his attempt at understanding the world of primary substances is a better epistemological route upon which to embark.
A literal reading of the Empedoclean principle, however, is not sufficient, Aquinas suggests, to discard it completely from an analysis of sensation and perception. By means of the act/potency distinction, both Aristotle and Aquinas provide a conceptual rehabilitation of Empedocles’s principles.
Now since these problems are insurmountable if the sense faculty consists of its objects in their actuality (as the early philosophers thought), Aristotle concludes that the sensible soul is clearly not actually, but only potentially the sense object. That is why sensation will not occur without an exterior sense-object, just as combustible material does not burn of itself, but needs to be ignited by an exterior agent. Whereas if it were actually fire, it would burn simply of itself. (Commentary on the Soul, no. 354; emphasis added)
Aquinas amplifies these suggestions:
Aristotle shows how it follows from the above that the old theory that ‘like senses like’ cannot be true. Everything potential, he says, is acted upon and moved by some active agent already existing; its actualizing function makes the potential thing like itself. In some sense, then, a thing is acted upon by both its like and its unlike. In the beginning, while the transforming process is going on, there is dissimilarity. At the end, however, when the thing is transformed and changed, there is similarity. And so it is between the sense faculty and its object. The early philosophers [antiqui] went wrong because they missed this distinction. (no. 357)
Given this argument, Aquinas believes he has reconciled the principles of Empedocles so that a viable philosophical analysis of sensation might follow. This rehabilitation is a necessary condition for the development of a thesis of intentionality in Aquinas’s philosophy of mind. Keeping the strict materialism inherent in Empedocles precludes, Aquinas believes, the possibility of an intentionality theory.
-  This proposition indicates what Nussbaum suggests about the incommensurability of ‘goods’ or ‘ends’in Aristotle’s philosophical theories; an end is the completion of a potency, and there are as many kinds ofends as there are distinct potencies.
-  In contemporary philosophy-of-mind discussions, the theory of Empedocles appears reducible to akind of ‘physicalism’ similar to the materialist theory propounded by J. J. C. Smart and D. M. Armstrong,among others. Known sometimes as the ‘Australian Theory’, physicalism, according to Smart, proposes thefollowing about the nature of mind: ‘The sciences of biology and psychology [...] are an application ofphysics and chemistry to natural history [. and] organisms are simply very complicated physico-chemicalmechanisms’: J. C. C. Smart, quoted in Jenny Teichman, Philosophy and Mind (Oxford: Blackwell, 1988),13. Teichman notes that Smart once argued that ‘everything’ falls under the rubric of physical explanation,‘except (Smart continues) the occurrence of sensations seems to me to be frankly unbelievable’.
-  This is similar structurally to Descartes’s ‘dream problem’ in the First Meditation.