Philosophical Foundations of Neuroscience. P. M. S. Hacker
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primarily to produce in it a certain very fine wind (that is “composed of very small, fast-moving particles”), or rather a very lively and pure flame, which is called the animal spirits.’48 This is an unfortunate name as it is not an apt descriptive term for components of a mechanical theory, for the word ‘spirit’ can be interpreted as a principle of life that animates the body or as the active principle of a substance extracted as a liquid. However, Descartes is quite explicit that ‘animal spirits’ are material: namely, ‘a certain very fine air or wind,’49 and that
what I am calling ‘spirits’ here are merely bodies; they have no property other than that of being extremely small bodies which move very quickly, like the jets of flame that come from a torch. They never stop in any place, and as some of them enter the brain’ s cavities, others leave it through the pores in its substance. These pores conduct them into the nerves, and then to the muscles. In this way the animal spirits move the body in all the various ways it can be moved.50
Descartes thus argued that the flow of animal spirits from the ventricles (in the case of motor action) involved the opening of particular valves in the walls of the ventricles, with a consequent flow of spirits into the appropriate motor nerve and contraction of muscle. In the case of involuntary behaviour associated with, for example, a pinprick, this would lead to a tension on just those filaments which open the appropriate valves in the walls of the ventricles to release the animal spirits into the motor nerves that contract the muscle to move the limb away from the point of the indentation.
Transmission involves inhibitory and excitatory processes
Descartes used the word ‘reflex’ only once in developing his conception of non-human animals as automata, although it is implied throughout his descriptions of animal behaviour and human non-volitional reactions. Although Descartes does not quote Fernel’ s Physiologia in his Treatise on Man, it is clear that his development of the doctrine of mindless motor behaviour in humans and animals has for its foundations the concept of the reflex first enunciated by Fernel.51 Treatise on Man argues that such motor behaviour requires not only an excitatory process, but also an inhibitory one, a speculation that was later to be confirmed experimentally by Sherrington and analysed at the cellular level by his student John Eccles. Descartes then argued that the excitatory and inhibitory processes, when acting together, allow animals and the bodies of human beings (when functioning independently of the intervention of the mind) to be described as automatons.
These suggestions of Descartes’s about the mechanism of reflex or involuntary movement, involving as they do the animal spirits stored in the ventricles, raise the question of the mechanism of voluntary movement. Here Descartes departed fundamentally from the ventricular doctrine. He denied that the ventricles are the seat of the sensitive and rational (including volitional) powers of human beings. He also denied that non-human animals have any sensitive powers in the sense in which human beings do, inasmuch as they lack consciousness. And he held that the human mind or soul interacts with the body in the pineal gland, which he incorrectly placed inside the ventricles.52
The pineal gland as locus for the sensus communis and point of interaction of mind and body
It is interesting to note the reason (or part of the reason) why Descartes concluded that the pineal gland is the locus of the sensus communis and of interaction between the body and the soul. It was because it is located between the two hemispheres of the brain and is not itself bifurcated. Consequently, he reasoned, it must be in the pineal gland that ‘the two images coming from a single object through the two eyes, or the two impressions coming from a single object through the double organs of any other sense [e.g. hands or ears] can come together in a single image or impression before reaching the soul, so that they do not present it with two objects instead of one.’53 These images or figures ‘which are traced in the spirits on the surface of the gland’ are ‘the forms of images which the rational soul united to this machine [i.e. the body] will consider directly when it imagines some object or perceives it by the senses’.54
It is noteworthy that Descartes warned that although the image generated on the pineal gland does bear some resemblance to its cause (immediately, the retinal excitation; mediately, the object perceived), the resultant sensory perception is not caused by the resemblance. For, as he observed, that would require ‘yet other eyes within our brain with which we could perceive it’.55 Rather, it is the movements composing the image on the pineal gland which, by acting directly on the soul, cause it to have the corresponding perception.
Descartes’ s conceptual error of attributing seeing to the soul rather than to the person or living being
The warning was apt, but the caution insufficient. Descartes was, of course, wrong to identify the pineal gland as the locus of a sensus communis, and wrong to think that an image corresponding to the retinal image (and hence to what is seen) is reconstituted in the brain. These are factual errors, and it is noteworthy that they still have analogues in current neuroscientific thought – in particular, in the common characterization of the so-called binding problem (discussed below, §5.2.3). But Descartes was right to caution that whatever occurs in the brain that enables us to see whatever we see, our seeing cannot be explained by reference to observation of such brain events or configurations. For, as he rightly observed, that would require ‘yet other eyes within our brain’. Nevertheless, he was confused – conceptually confused – to suggest
1 that images or impressions coming from double organs of sense must be united in the brain to form a single representation in order that the soul should not be presented with two objects instead of one;
2 that the soul ‘considers directly’ the forms or images in the brain when it perceives an object; and
3 that it is the soul, rather than the living animal (human being), that perceives.The first error presupposes precisely what he had warned against, for only if the images or impressions were actually perceived by the soul would there be any reason to suppose that the ‘two images’ would result in double vision or double hearing. The second error is the incoherence of supposing that in the course of perceiving, the soul or mind ‘considers’ anything whatsoever (no matter whether forms or images) in the brain. And the third is the error of supposing that it is the soul or mind that perceives. (It is instructive to contrast this idea with Aristotles’ denial that it is the psuchē that is angry, pities, learns or thinks.) We have already noted the much earlier occurrence of this confusion in Nemesius. The mistake is a form of a mereological fallacy (mereology being the logic of the relations between parts and wholes). For it consists in ascribing to a part of a creature attributes which logically can be ascribed only to the creature as a whole.56 The particular form which this mereological fallacy took in Descartes consisted in ascribing to the soul attributes which can be ascribed only to the whole animal. We shall discuss this matter in detail in chapter 3.
By the middle of the seventeenth century, Descartes had replaced the ventricular doctrine localizing psychological functions in the ventricles of the brain with his idiosyncratic interactionist doctrine localizing all psychological functions in the pineal gland, which he conceived to be the point of interaction between mind and brain. This is how he met Vesalius’ s objection that it is difficult to reconcile the idea that the different ventricles are associated with different cognitive and cogitative powers when the ventricles of humans are so similar to those of other mammals. Furthermore, he had replaced psychic pneuma by animal spirits as the medium through which the pineal gland produces its effects. This amounted to replacing the fluid derived from the pneuma described by Aristotle with mechanical corpuscles that possessed special properties. However, his contemporaries were soon to point out that the pineal gland is not inside the ventricles and, furthermore, that since other mammals too possessed this gland, his response to Vesalius was inadequate.
Descartes’ s primary contribution
Nevertheless, Descartes had made the fundamental contribution of