Marsilio Ficino’s Platonic Theology, Volume II

23Because sleep often overcomes us against our will, must we fear that some comparable failure could convert the mind into body? That would be as though, because our limbs grow slack in sleep, the mind itself became weaker in some part. It does not, to be sure, perceive sensible things while asleep, because whatever causes sleep comes from the body and operates in the body. It lulls and in a sense closes the bodily senses. Yet the soul yields willingly to this bodily change, since the change accords with nature and restores the body after its labors.

24Sleep does not, however, take from the mind either its power of sensing or its power of understanding. It has images of sensible things immediately present with so vivid a likeness that at the time they cannot be distinguished from the things whose images they are. And whatever it understands is no less true for the sleeper than for one awake. Suppose, for example, that someone dreams he is conducting an argument and, by following true principles, learns something in the course of it. On waking, those same immutable truths remain, although everything else proves false: the place where the argument seemed to occur, the person with whom it seemed to be conducted, the very words belonging to the dream through which the argument seemed to proceed, and other things of this sort. Such things are perceived by the senses and enacted by waking people, yet they pass away and in no respect attain the everlasting presence of true principles. From this we conclude that the bodily change called sleep can diminish not the power of the soul itself but only the life of the body.