To the Stoical opinion Descartes much inclines. He affirms that the soul or mind is united specially to a certain part of the brain called the pineal gland, which the mind by the mere exercise of the will is able to move in different ways, and by whose help the mind perceives all the movements which are excited in the body and external objects. This gland, he affirms, is suspended in the middle of the brain in such a manner that it can be moved by the least motion of the animal spirits. Again, he affirms that any variation in the manner in which the animal spirits impinge upon this gland is followed by a variation in the manner in which it is suspended in the middle of the brain, and moreover that the number of different impressions on the gland is the same as that of the different external objects which propel the animal spirits toward it. Hence it comes to pass that if the gland, by the will of the soul moving it in different directions, be afterwards suspended in this or that way in which it had once been suspended by the spirits agitated in this or that way, then the gland itself will propel and determine the animal spirits themselves in the same way as that in which they had before been repelled by a similar suspension of the gland. Moreover, he affirmed that each volition of the mind is united in Nature to a certain motion of the gland. For example, if a person wishes to behold a remote object, this volition will cause the pupil of the eye to dilate, but if he thinks merely of the dilation of the pupil, to have that volition will profit him nothing, because Nature has not connected a motion of the gland which serves to impel the animal spirits towards the optic nerve in a way suitable for dilation or contraction of the pupil with the volition or dilation or contraction, but only with the volition of beholding objects afar off or close at hand. Finally, he maintained that although each motion of this gland appears to be connected by Nature from the commencement of our life with an individual thought, these motions can nevertheless be connected by habit with other thoughts, a proposition which he attempts to demonstrate in his "Passions of the Soul" (art. 50, pt. 1).