Lives and Opinions of Eminent Philosophers

​Of impressions, according to them, some are sensory and others are not: sensory are those received through a sense organ or sense organs, and non-sensory are those received through the intellect, such as those of incorporeals and of other things received by reason. Of the sensory impressions, 〈some〉 arise from what exists, accompanied by yielding and assent. There are also among impressions appearances which arise as if from what exists. Furthermore, of impressions some are rational and others irrational: rational are those of rational animals, and irrational are those of irrational animals. The rational ones, then, are thoughts, while the irrational ones have obtained no name. And some are technical, while others are non-technical; at any rate, a picture is viewed in one way by a craftsman and in another way by a layman.

​Sensation is spoken of by the Stoics as the breath extending from the ruling part to the senses, and as the comprehension achieved through them, and as the constitution regarding the sense organs according to which some people become disabled. The activity itself is also called sensation. Comprehension, according to them, arises by sensation in the case of white and black, rough and smooth, but by reason in the case of conclusions drawn through demonstration, such as that gods exist and that they exercise providence. For of the things conceived, some were conceived by direct encounter, others by similarity, others by analogy, others by transposition, others by composition, and others by contrariety.

​By direct encounter, then, sensible things were conceived; by similarity, things derived from something at hand, as Socrates is conceived from his portrait; by analogy, either augmentatively, as Tityos and the Cyclops, or diminutively, as the Pygmy. The center of the earth, too, was conceived by analogy from smaller spheres. By transposition, for example, eyes upon the chest; by composition, the Centaur was conceived; and by contrariety, death. Conception also takes place by a kind of transition, as in the case of things expressible and space. Something just and good is conceived naturally; and by privation, for instance, a man without hands. Such are the doctrines they hold concerning impression, sensation, and thought.

​They say that cognitive impression—that is, the one arising from what exists—is the criterion of truth, as Chrysippus says in the twelfth book of his Physics, and Antipater and Apollodorus. For Boethus leaves multiple criteria: intellect, sensation, desire, and knowledge; while Chrysippus, disagreeing with himself, says in the first book of On Reason that the criteria are sensation and preconception; and a preconception is a natural conception of universals. Some others of the older Stoics leave right reason as the criterion, as Posidonius says in On the Criterion.

​It is agreed by most that the study of dialectic begins from the topic concerning voice. And voice is air that has been struck, or the proper sensible object of hearing, as Diogenes of Babylon says in his Art of Voice. The voice of an animal is air struck by an impulse, but that of a human being is articulate and emitted from the intellect, as Diogenes says, which reaches maturity from the age of fourteen. And voice is also a body according to the Stoics, as Archedemus says in On Voice, and Diogenes, Antipater, and Chrysippus in the second book of his Physics.