Mathematical Mirror

that the brain is enveloped by a double membrane: the one is called the pia mater, which is the one immediately containing the brain, and the other is called the dura mater, which adheres to the concavity of the bone of the head, which is called the skull. For the latter is harder so that it may resist the bone, and the other is softer and more delicate on account of the softness of the brain, whose substance is medullary and has three distinctions, which are called chambers, cells, and parts. And the divisions in the first cell are two virtues: one is the sensus communis common sense residing in its anterior part, as Avicenna says in Book 1 On the Soul, which is like a fountain in respect to the particular senses, and like a center in respect to lines extending from it to the circumference, according to Aristotle in Book 2 On the Soul, who judges regarding individual sensible things. For a judgment regarding a visible thing is not completed before the species comes to the common sense, and so it is with the audible and others, as is written at the end of On Sense and the Sensible and in Book 2 On the Soul. It judges the diversity of sensible things, such as the fact that in milk, white is different from sweet—a thing which neither vision nor taste can do, because they do not discern the extremes, as Aristotle maintains in Book 2 On the Soul. And it judges the operations of the particular senses, for vision does not feel itself seeing, nor does hearing perceive itself hearing, but another virtue—which is the common sense—does, as Aristotle maintains in Book 2 On Sleep and Waking. Its ultimate operation, however, is to receive the species coming from the particular senses and to complete the judgment regarding them. But it does not retain them on account of the excessive slipperiness of its instrument, according to what Avicenna wants in Book 1 On the Soul. And therefore, it is necessary that there be another virtue of the soul in the last part of the first cell, whose office is to retain the species coming from the particular senses on account of its tempered humidity and dryness; this is called Imaginatio Imagination. It is the chest and repository of the common sense, according to Avicenna, who provides the example of a seal: water receives its species well but does not retain it because of its superfluous humidity, whereas wax retains it well because of its tempered humidity combined with dryness.