The Stoics wrote numerous works on logic, but, unfortunately, none of these has survived; thus one must rely on summaries of their works and the occasional quotation from them. For this reason, it is impossible to formulate a complete "Stoic logic" even for individual Stoics. Stoic logic is broken into sub-categories. Diogenes explains, "Some divide the logical part of the system into the two sciences of rhetoric and dialectic; while some would add that which deals with definitions and another part concerning canons or criteria"� (Lives, 7.41). Rhetoric concerns "invention of arguments, their expression into words, their arrangement, and delivery," among other things.�The other parts of logic, which some Stoics subsumed under the single heading of dialectic, concern formal logic, epistemology and theory of language, to use modern terminology. From the available sources,�the Stoics contributed little that was new to the study of formal logic; they merely modified Aristotle's contribution.
As most Greeks, the Stoics believed that a human being had a soul. For the Stoics, the soul was corporeal, and was diffused throughout the body. (The individual soul was actually a part of the world-soul.)�Diogenes says, "And the soul is a nature capable of perception. And they regard it as a breath of life, congenital with us; from which they infer that it is a body and secondly that it survives death" (Lives, 156). The soul is that which comes into contact with objects outside the perceiver by means of the five senses, which are called parts [or better "functions"] of the soul.�The perception of an object by the soul through one of the five senses the Stoics called "presentation" (phantasia). Diogenes explains, "A presentation is an imprint on the soul; the name having been appropriately borrowed from the imprint made by the seal upon the wax" (Lives, 7.45). The soul is like a wax tablet and the object perceived is like a seal that impresses a copy of itself into the wax. (Chrysippus warns, however, that one should not think that literally an object impresses itself upon the soul [Lives, 7.50].)� Diogenes quotes Diocles the Magnesian concerning the importance of "presentation" in Stoic philosophy:
The Stoics agree to put in the forefront the doctrine of presentation and sensation, inasmuch as the standard by which the truth of things is tested is generically a presentation, and again the theory of assent and that of apprehension and thought, which precedes all the rest, cannot be stated apart from presentation. For presentation comes first; then thought, which is capable of expressing itself, puts into the form of a propositions that which the subject receives from a presentation (Lives, 7. 49)
��� The Stoics, however, made a distinction between two types of presentation; the one is a sensation...