Descartes is infamous for his mind/body dualism. It is Descartes’ dualism that makes his philosophy memorable to the twentieth first century reader, but during the seventeenth century it was the nature and extent of his materialism and mechanism that raised eyebrows and considerable criticism. As Dennis Des Chene notes in his latest book, Spirits and Clocks (S&C), Descartes wrote more about the body than he did about the mind, although Descartes suppressed this material for fear of persecution. Yet Descartes’ physiology has not received a great deal of attention. Spirits and Clocks provides a welcome corrective to this state of scholarship.

Descartes’ physiology is uncompromising. The internal self-motion and equilibrium of the body and the generation of the individual from the seed are all explained, thought Descartes, solely by the laws governing the motions of matter. But this was outrageous – the received Aristotelian tradition held to a tripartite soul. Growth, nutrition, reproduction, the motions of the heart and the formation of the fetus required the activity and direction of the soul, as did all vital motion. The body and its organs may be a machine or instrument of the soul, agreed the Aristotelians, but an instrument that receives direction from that soul. But Descartes claims that the body is a machine in a different, although to us familiar sense: it is a lifeless machine lacking any substantial principle directing it. Descartes’ dualism of mind and matter implies the rejection of the living as a natural kind. Nature cannot be divided into the living and the nonliving, but should rather be divided into the thinking and the unthinking. But if this radical reinterpretation of nature is to be successful, Descartes’ body-machine must simulate what has been lost.

It is the theme of simulation that begins Descartes’ Treatise on Man and guides the reader through S&C. Descartes asks us to conceive of a fictional world in which God makes machines to resemble us as much as possible. Descartes wants us to see that the success of this thought-experiment reveals the success of the mechanistic approach to the living. We will see no difference between these fictional animal-machines and the real things. Mechanism is therefore empirically adequate to the phenomena, without the need for the vegetative or sensitive soul of the Aristotelians. That it really is empirically adequate is the goal of Descartes’ physiology, but one that Des Chene and Descartes’ contemporaries judge a failure. By the end of the seventeenth century no-one could advocate the particular mechanisms Descartes had suggested, although the seeds of mechanism were slowly growing.

Des Chene explores how Descartes attempts to explain the motions attributed to the non-rational soul and the subtle conceptual shifts required in the attempt. A number of conceptual shifts must be made. He notes that the idea of a machine as an instrument must lose its association with the idea that machines are instruments of the soul. In Descartes view, machines can move and maintain themselves. Bodily organs, like the heart, must have their identity without reference to God’s intentions – his inscrutable final causes; thus putting strain on our natural talk of the functions and use of organs. Indeed, it is difficult, if not impossible, to recognize that a stomach or heart in a human and in a cow are the same organs, unless it is by their common functions. The unity of material bodies must also be explained without references to a form, or a presupposed notion of health or normality (which would assume a notion of unity). But where would this idea of health and normality come from, if indeed, bodies are merely material aggregates?

This last theme, that of the unity of the body, will attract those scholars concerned with the problematic union of body and soul in Descartes’ philosophy. Here Des Chene treats the soul’s relation to the body as analogous to the Aristotelian relation of form to matter. The body, once unified with the soul, has an intentional or functional unity; thus giving us a conception of its health relative to its success in accommodating the unification. But prior to that union the human body (and animal bodies) can have, at best, physical unity or identity: the unity of constrained activity given some initial force.

The status of the body’s identity is, of course, important for the interpretation of its union with the soul, and reveals a worry concerning this interpretation. If Descartes utilizes some notion of bodily identity independent of the identity the body gains upon being united with the soul, then the need to see the soul’s unification with the body as analogous to the Aristotelian unity of form with matter is surely diminished. This is because the Aristotelian soul is supposed to be responsible for making mere aggregates of matter into organs and things. If Descartes intends to accommodate a notion of unity for unensouled material bodies, (which he needs to do), then it is difficult to understand why he would intend to understand the soul’s union with the body on a broadly Aristotelian model. But Des Chene is not intending to sort out all the issues regarding the soul’s unity with the body: this is, after-all, a book primarily about the body.

Spirits and Clocks is a thorough, focused and meticulous study of Descartes’ physiology. Des Chene’s knowledge of the late Aristotelianism and early modern thought is impressive. Along with his detailed account of "Descartes’ body" Des Chene discusses the notions of mechanism and machine, the use of illustrations in natural philosophy and the role of fiction and simulation as modes of argument. S&C is, therefore, essential reading to Descartes scholars, and will be of considerable interest to historians of science and students of early modern thought.

Brian Garrett (Ph.D.) Dept. of Philosophy York University, Toronto.

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