Book Review

Aquinas's Theory of Perception:
An Analytic Reconstruction

Anthony J. Lisska
Oxford University Press (16 June 2016)
ISBN: 978-0198777908

Book CoverThis is a wonderfully engaging and detailed study of concept formation and abstraction through the agent intellect according to Aquinas. His account of the inner sense and the vis cogitativa are examined in order to determine how individual things are perceived according to Thomistic teaching. 

In the Summa Theologiae, it is suggested that the phantasia is identical with the imagination for Aquinas. In contrast, notes Lisska, in his Commentary on the Soul, the phantasia is identified with the imagination, the vis cogitativa and the sense memory. The imagination conserves the sensible forms received from the external senses through the common sense. The vis cogitativa is awareness of individual things. The sense memory conserves the acts of awareness in the vis cogitativa. The imagination is thus to the common sense as sense memory is to the vis cogitativa. The author notes that for Aquinas the vis cogitativa has two functions:to enable us to recognize individual things and to enable us to recognize individuals as members of a natural kind. For Aquinas, each individual thing is an instance of a natural kind, a substance, made of matter and form. How can we be made aware of these primary substances since we receive but the forms of things? It is through the complicated process known as abstraction that we are made aware of the substantial form. Abstraction involves the agent intellect and the possible intellect. But this does not answer the question: how do we know individual things as individual things? It is here that the vis cogitativa comes into play. It is this which enables us to distinguish perception from sensation according to Aquinas, says Lisska. It is this faculty of the inner sense by which we are made aware of individual things as members of a particular kind. This mental act is different from the mental act of concept formation, which is by abstraction through the agent intellect.

To know a concept is to have an awareness of the essence of a thing. It is the vis cogitativa which enables us to see individuals as members of a particular kind. It enables us to distinguish Peter from John, as two different individual persons.

The author thus notes that for Aquinas, there are two important intentional structures in his philosophy of mind: the agent intellect and the vis cogitativa. Both are needed in order for us to perceive essential properties in things.

The senses carry a message that they themselves cannot interpret. Gilson and other great Thomists suggested that it is the agent intellect that acts as interpreter. Lisska suggests that the vis cogitativa also needs to be taken into consideration. The agent intellect alone does not "know" a concept.

The author notes that thanks to Peter Geach and other great recent philosophers, Aquinas's philosophy of mind has been gaining recent popularity. Non-Thomistic philosophers including Anthony Kenny and Martha Nussbaum have praised it.

In conclusion, this is a beautifully written work on Aquinas's philosophy of mind. As the author notes, Aquinas's teaching on the vis cogitativa has not received much attention till now. 

Reviewed by Dr Pravin Thevathasan