century, with the advances in logic, and the modern propositional calculus, that it became clear that Stoic logic constituted a significant achievement. Epistemology For the Stoics, truth is distinguished from error by the sage who possesses right reason. Chrysippus's 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; that is, the soul receives a modification from every external object that acts upon it, just as the air receives countless strokes when many people are speaking at once. In the receipt of an impression, the soul is purely passive and the impression reveals not only its own existence, but that also of its causejust as light displays itself and the elements that are in it. The power to name the object resides in the understanding. First must come the impression, and the understandinghaving the power of utteranceexpresses in speech the affection it receives from the object. True presentations are distinguished from those that are false by the use of memory, classification and comparison. If the sense organ and the mind are healthyand provided that an external object can be really seen or heardthe presentation, due to its clearness and distinctness, has the power to extort the assent that always lies in our power, to give or to withhold. In a context in which people are understood to be rational beings, reason is developed out of these notions. Physics Chrysippus insisted on the organic unity of the universe, as well as the correlation and mutual interdependence of all of its parts. He said, the universe is "the soul and guide of itself." Following Zeno, Chrysippus determined fiery breath or aether to be the primitive substance of the universe. Objects are made up of inert formless matter and an informing soul, "pneuma", provides form to the undifferentiated matter. The pneuma pervades all of substance and maintains the unity of the universe and constitutes the soul of the human being. The classical elements change into one another by a