This page will lead you to some of the illustrations included in Clerselier’s posthumous edition of the Treatise. Presumably they were copied from rough sketches in Descartes’ manuscript.

I should point out that the illustrations are not anatomically very accurate, and they exaggerate the size of the pineal gland and the internal cavity of the brain. But not bad, considering that it was illegal to dissect corpses in those days.

The first illustrations show the position of the pineal gland from different angles, with examples of animal spirits radiating to or from it.

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The remaining figures are mainly concerned with how vision projects images onto the surface of the pineal gland; how information from different senses are combined into a single ‘common sense’ (looking at an arrow while smelling a flower); and how sensory information is co-ordinated with muscular action.

Note figure 29 in particular. Here Descartes naively tries to solve the problem of how we see things the right way up, given that the images on the retina are upside-down (as indeed they are). He makes the animal spirits cross over inside the brain so as to reverse the effect in the lens of the eye. In my view, this provides very strong evidence that Descartes had a picture of the soul sitting on the pineal gland looking at images inside the head. If so, he hasn’t really explained perception at all, since it is more of a problem how the soul perceives images, than how the brain receives them from external objects.

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