DIADUMENUS. And do they not also determine the substance and generation of conception itself, even against the common conceptions? For conception is a certain imagination, and imagination an impression in the soul. Now the nature of the soul is an exhalation, in which it is difficult for an impression to be made because of its tenuity, and for which it is impossible to keep an impression it may have received. For its nutriment and generation, consisting of moist things, have continual accession and consumption. And the mixture of respiration with the air always makes some new exhalation, which is altered and changed by the flux of the air coming from abroad and again going out. For one may more easily imagine that a stream of running water can retain figures, impressions, and images, than that a spirit can be carried in vapors and humors, and continually mingled with another idle and strange breath from without. But these men so far forget themselves, that, having defined the conceptions to be certain stored-up intelligences, and memoirs to be constant and habitual impressions, and having wholly fixed the sciences, as having stability and firmness, they presently place under them a basis and seat of a slippery substance, easy to be dissipated and in perpetual flux and motion.