Lisska states in his introduction, that ‘the teleological principle on which this study has been undertaken’ is ‘to offer a substantive yet critical analysis of Aquinas's significant account of inner sense under the general rubric of sensation and perception’. Lisska is principally motivated by a desire to ‘eliminate some of what Kenny called the ‘puzzling’ issues in Aquinas's philosophy of mind’. As for his intended audience, Lisska hopes his monograph will be of benefit ‘both to novices coming to the work of Aquinas with little background in medieval philosophy and to academically trained philosophers and also historians of psychology generally interested in medieval theories of mind’. In this regard, Lisska is very successful – his book is accessible and well written; at the same time, he engages seriously with the many possible interpretations of Aquinas's theory of perception, and he provides some helpful comparisons with other important theories of perception.
One of the principal differences between Aquinas's theory of perception and more modern theories is that Aquinas is not interested in justifying the possibility of knowledge; rather, given that we know things, Aquinas sets out to show how this knowledge is possible. Lisska explains that the crucial principle in Aquinas's theory of perception is that the same form of the thing known is in the knower. However, the form's mode of existence in the knower is different from the form's mode of existence in the thing known: in the knower, the form has esse intentionale, whereas in the thing known, it has esse naturale. By maintaining a strict formal identity between the mental act of the knower and the thing known, and by differentiating the corresponding modes of existence, Aquinas's theory of perception is not limited to a representative theory of perception; for Aquinas, the knower genuinely perceives the external thing existing in the world rather than just an internal representation of the thing existing in the mind. Thus, Aquinas's theory is not plagued by the problems of scepticism inherent in representative theories of perception.
Lisska goes on to give an account of the five exterior senses and the four interior senses. According to Aquinas, sensation requires a sensible object, a sense faculty, and a medium. In the case of visual perception, Lisska describes in some depth this triadic relation between object, sense faculty and medium. This account presents some of the challenges in reconciling Aquinas's theory of sensation with modern understandings of physics. Aquinas rejected the notion of atoms in a void. The diaphanous medium had to be some kind of body such as air. Aquinas thought the colour of the sensible object affected the transparent medium which in turn affected the sense faculty. Since Aquinas had no understanding of ‘action at a distance’, this process of sensation all had to take place through actual contact.
Despite the disparity between Aquinas's theory of sensation and modern physics, Lisska claims that Aquinas was providing a ‘metaphysical rather than a physical account of change’. It is not entirely obvious what Lisska means by making this distinction. Following Haldane, Lisska claims that ‘Aquinas's causality is not reducible to efficient cause but is instead an instance of formal causality’. Thus, one might suppose a ‘metaphysical account of change’ is one that considers formal causality, whereas a ‘physical account of change’ considers efficient causality. However, such an imposition seems inconsistent with Aquinas's own understanding of physics. For Aquinas physics was the science of mobile being, and this science appealed to all four causes in its demonstrations. By dismissing any talk of efficient causality, one is in danger of making Aquinas's theory of perception just as subject to the problem of scepticism as contemporary theories are. After all, if one has no account of the efficient causes by which there is a formal identity between the mind of the knower and the sensible object, why should there be any identity at all? If we ignore efficient causality, we do not have an account of how direct realism is possible.
The role of medieval physics also plays an important role in Aquinas's treatment of the four interior senses: the sensus communis, the imagination, the aestimative (or the vis cogitativa) and the memorative powers. Aquinas explains that the sensitive soul must have the power to receive sensible species and the power to retain them, the former power being the common sense, and the latter power being the imagination. Likewise, the powers to receive intentions and to retain intentions belong to the vis cogitativa and the memorative powers respectively. Lisska, however, chooses to omit Aquinas's explanation of why a receptive power is necessarily distinct from a retentive power, namely because ‘to receive and retain are, in corporeal things, reduced to diverse principles; for moist things are apt to receive, but retain with difficulty, while it is the reverse with dry things’. (ST I q. 78. a. 4). Lisska notes that Frede finds Aquinas's account of the vis cogitative an embarrassment because of the way it hovers between sense‐perception and thought, and Lisska does a very good job in responding to Frede's accusation. However, it seems rather ironic that Lisska never speaks of how embarrassed one might be for Aquinas when Aquinas distinguishes the interior senses in terms of moistness and dryness. Unless one raises the question of this embarrassment, one will be unable to consider whether anything needs to be or can be done about it. For instance, rather than dismissing Aquinas's understanding of moistness and dryness, perhaps we need to rediscover these principles on a more foundational level in the context of modern physics. Or maybe Aquinas's theory of perception is so deeply coupled to his outdated physics, that his theory is beyond redemption.
Lisska set out to eliminate some of the ‘puzzling’ issues in Aquinas's philosophy of mind, and in many ways, he has been successful. Nevertheless, there remain many puzzles that need to be resolved if Aquinas's theory of perception is to provide a convincing alternative to other contemporary theories.
