Logic and Language
The established doctrine that Aristotle invented formal logic has sometimes been disputed, rather unwisely, on Plato's behalf; but the truth is that there is no evidence in Plato's writings that the notion of formal logic ever occurred to him, and he made no contribution to it. The formal logician investigates the force of certain words (the so-called logical constants such as all, some, a, none, not, if, and, or) and he exhibits the force of these words in terms of each other by constructing models of statements and saying how they are related. Thus he tells us that "All S is P" entails "Some S is P" and is incompatible with "This S is not P" and so on. The idea of doing this first occurred to Aristotle (though he would not perhaps have described it in the terms we have used). It does not seem to have occurred to Plato.