CORRSPONDENCE WITH MERSENNE
Translation © George MacDonald Ross, 1975–1999
Descartes to Mersenne, 30th July, 1640
[123] I agree that the soul can perfectly well use double parts, and that it also uses the spirits, which cannot all reside in this [124] gland; for I certainly do not imagine that the soul is contained in the gland in such a way that it does not extend its activity elsewhere. But it is one thing to use, and a completely different thing to be immediately joined or united. Since our soul is in no way double, but one and indivisible, it seems to me that the part of the body to which it is most immediately united must also be one and not divided into two similar things; and this gland is the only thing of such a kind that I can find in the whole of the brain.
Descartes to Mersenne, 24th December, 1640(?)
[263] We know by the experience of imagining that the seat of the common sense, that is to say the part of the brain in which the soul exercises all its principal functions, must be mobile. . . . . [264] Now it is very reasonable that the conarium should be similar to a gland, because the principal function of all glands is to receive the most subtle parts of the blood which are exhaled from the vessels surrounding them, and the function of the conarium is to receive the animal spirits in the same way. And inasmuch as it is the only unique solid part of the whole brain, it must necessarily be the seat of the common sense, that is to say of thought, and consequently of the soul; for the one cannot be separated from the other. Otherwise it would have to be admitted that the soul is not immediately united to any solid part of the body, but only to the animal spirits which are in its cavities, and which continually enter and leave them like the water of a river — but this would be considered too absurd. . . .
[265] As for your comment that I have not said a word [266]about the immortality of the soul, you should not be surprised at that; for I could hardly demonstrate that God is unable to annihilate it, but only that it is of a nature entirely distinct from that of the body, and consequently that it is not naturally subject to death along with the body. This is all that is required for establishing religion; and it is also all that I set myself to prove.
Descartes to Mersenne, 21st April, 1641
[361] It is in different senses that I include and exclude imaginations in the definition of cogitatio or ‘thought’. That is, corporeal forms or species, which we must have in the brain in order to imagine anything, are not thoughts. Thought is the operation of the mind which imagines, i.e. which directs itself towards these species. . . . [n.1]
[You have already written to me about the objection] that no nerve goes to the conarium, and that it is too mobile to be the seat of the common sense. But these two facts are entirely in my favour. Since every nerve leads to some sense-organ or particular movement (some to the eyes, others to the ears, arms, &c.), if one rather than the others led to the conarium, it would follow that it was not the seat of the common sense, since all must lead to it in the same way; and it is [362] impossible for them all to lead there other than by the agency of the spirits, as in fact happens with the conarium. It is also certain that the seat of the common sense must be extremely mobile in order to register all the impressions which come from the senses; but it must be such that it cannot be moved except by the spirits which transmit these impressions; and only the conarium is of this sort.