XXXVI. And, that we may also enter into some more minute details respecting them, we will subjoin what refers to what they call their introductory science, as it is stated by Diocles, of Magnesia, in his Excursion of Philosophers, where he speaks as follows, and we will give his account word for word. The Stoics have chosen to treat, in the first place, of perception and sensation, because the criterion by which the truth of facts is ascertained is a kind of perception, and because the judgment which expresses the belief, and the comprehension, and the understanding of a thing, a judgment which precedes all others, cannot exist without perception. For perception leads the way; and then thought, finding vent in expressions, explains in words the feelings which it derives from perception. But there is a difference between φαντασία and φάντασμα. For φάντασμα is a conception of the intellect, such as takes place in sleep; but φαντασία is an impression, τύπωσις, produced on the mind, that is to say, an alteration, ἀλλοίωσις, as Chrysippus states in the twelfth book of his treatise on the Soul. For we must not take this impression to resemble that made by a seal, since it is impossible to conceive that there should be many impressions made at the same time on the same thing. But φαντασία is understood to be that which is impressed, and formed, and imprinted by a real object, according to a real object, in such a way as it could not be by any other than a real object; and, according to their ideas of the φαντασίαι, some are sensible, and some are not. Those they call sensible, which are derived by us from some one or more senses; and those they call not sensible, which emanate directly from the thought, as for instance, those which relate to incorporeal objects, or any others which are embraced by reason. Again, those which are sensible, are produced by a real object, which imposes itself on the intelligence, and compels its acquiescence; and there are also some others, which are simply apparent, mere shadows, which resemble those which are produced by real objects. Again, these φαντασίαι are divided into rational and irrational; those which are rational belong to animals capable of reason; those which are irrational to animals destitute of reason. Those which are rational are thoughts; those which are irrational have no name; but are again subdivided into artificial and not artificial. At all events, an image is contemplated in a different light by a man skilful in art, from that in which it is viewed by a man ignorant of art.