Lives and Opinions of Eminent Philosophers

​There are three forms of knowledge: one practical, another productive, and a third theoretical. Architecture and shipbuilding are productive; for in their case it is possible to see a work that has been produced. Politics, flute-playing, lyre-playing, and the like are practical; for it is impossible to see any work produced by them, but they perform an action: for one plays the flute and the lyre, and another takes part in politics. Geometry, harmonics, and astronomy are theoretical; for they neither act nor produce anything, but the geometer contemplates how lines are related to one another, the harmonist contemplates sounds, and the astronomer contemplates the stars and the universe. Of forms of knowledge, then, some are theoretical, some practical, and some productive.

​There are five forms of medicine: pharmaceutical, surgical, dietetic, diagnostic (nosognomonic), and auxiliary (relieving). Pharmaceutical medicine cures infirmities through drugs; surgical medicine heals through cutting and cauterizing; dietetic medicine dispels infirmities through regulating lifestyle; diagnostic medicine through discerning the illness; and auxiliary medicine, by assisting immediately, relieves pain. Of medicine, then, one part is pharmaceutical, another surgical, a third dietetic, a fourth auxiliary, and a fifth diagnostic.

​There are two divisions of law: written and unwritten. That by which we conduct civic life in the cities is written. That which arises according to custom is called unwritten; such as not going into the marketplace naked, nor putting on a woman's cloak. For no written law forbids these things, but nevertheless we do not do them because they are forbidden by unwritten law. Of law, then, there is the written and the unwritten. Discourse is divided into five kinds, one of which is that which politicians deliver in assemblies, which is called political.

​Another division of discourse is that which orators write and deliver for display, for encomia, censures, and accusations; such a species is rhetorical. A third division of discourse is that which private persons converse in with one another; this manner is called private. Another division of discourse is that in which people converse by asking questions briefly and answering the questioners; this discourse is called dialectical. A fifth division of discourse is that which craftsmen hold concerning their own art; this is called technical. Of discourse, then, one part is political, another rhetorical, another private, another dialectical, and another technical.

​Music is divided into three parts: one is by the mouth alone, such as song; secondly, by the mouth and the hands, such as singing to the lyre; and thirdly, from the hands alone, such as lyre-playing. Of music, then, one kind is from the mouth alone, another from the mouth and the hands, and another from the hands. Good birth is divided into four forms. One is if the ancestors are noble and good and just; people say that those born from them are well-born. Another is if the ancestors were rulers and magistrates; people say that those born from them are well-born. Another is if the ancestors are renowned, for instance from generalship or from the crown games; for we also call those born from them well-born.