Epistemology
For the Stoics, truth is distinguished from error by the sage who possesses right reason. Chrysippus' theory of knowledge was empirical. The senses transmit messages from the external world, and their reports are controlled not by referring them to innate ideas, but by comparing them to previous reports stored in the mind. Zeno had defined impressions of sense as "an impression in the soul", and this was interpreted literally by Cleanthes, who compared the impression on the soul to the impression made by a seal on wax. Chrysippus preferred to regard it as an alteration or change in the soul. The soul receives a modification from every external object which acts upon it, just as the air receives countless strokes when many people are speaking at once. In receiving an impression the soul is purely passive and the impression reveals, not only its own existence, but that also of its cause, just as light displays itself and the things that are in it. The power to name the object resides in the understanding. First must come the impression, and the understanding, having the power of utterance, expresses in speech the affection it receives from the object. True presentations are distinguished from false by the use of memory, classification and comparison. Provided the sense organ and the mind be healthy, provided an external object be really seen or heard, the presentation, in virtue of its clearness and distinctness, has the power to extort the assent which it always lies in our power to give or to withhold. Reason, in virtue of which people are called rational beings, is developed out of these notions.
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