Descartes’ short remarks about a small gland in the middle of the brain which is of paramount importance apparently generated a lot of interest. In 1640, Descartes wrote several letters to answer a number of questions that various persons had raised. In these letters, he not only identified the small gland as the conarion or pineal gland (29 January 1640, AT III:19, CSMK 143), but also added some interesting points to the Treatise of man. First, he explained why he regarded it as the principal seat of the rational soul (a point that he had not yet addressed in the Treatise of man): “My view is that this gland is the principal seat of the soul, and the place in which all our thoughts are formed. The reason I believe this is that I cannot find any part of the brain, except this, which is not double. Since we see only one thing with two eyes, and hear only one voice with two ears, and in short have never more than one thought at a time, it must necessarily be the case that the impressions which enter by the two eyes or by the two ears, and so on, unite with each other in some part of the body before being considered by the soul. Now it is impossible to find any such place in the whole head except this gland; moreover it is situated in the most suitable possible place for this purpose, in the middle of all the concavities; and it is supported and surrounded by the little branches of the carotid arteries which bring the spirits into the brain” (29 January 1640, AT III:19–20, CSMK 143). And as he wrote later that year: “Since it is the only solid part in the whole brain which is single, it must necessarily be the seat of the common sense, i.e., of thought, and consequently of the soul; for one cannot be separated from the other. The only alternative is to say that the soul is not joined immediately to any solid part of the body, but only to the animal spirits which are in its concavities, and which enter it and leave it continually like the water of river. That would certainly be thought too absurd” (24 December 1640, AT III:264, CSMK 162). Another important property of the pineal gland, in Descartes’ eyes, is that it is small, light and easily movable (29 January 1640, AT III:20, CSMK 143). The pituitary gland is, though small, undivided and located in the midline, not the seat of the soul because it is outside the brain and entirely immobile (24 December 1640, AT III:263, CSMK 162). The processus vermiformis of the cerebellum (as Descartes called the appendage which Galen had discussed) is not a suitable candidate because it is divisible into two halves (30 July 1640, AT III:124, not in CSMK).