The architectonic of the Nicomachean Ethics, as that has come down to us, corresponds closely to that of Plato's Republic. That the substance of the Aristotelian ethic, both in its main orientations and in the texture of its argumentation, is Platonic through and through, is of course not in dispute. What appears not to have been noticed is the correspondence between the ways in which the two works are organized. The correspondence is so extensive that it seems hard to doubt that Aristotle (or whoever put the Ethics together) hung his work on Plato's armature. If that is the case, much light should be shed on the later project by divergences between the tactics of the two works at the main points of coincidence. On the assumption that the Republic is a highly structured work and the Ethics is not evidently so, the procedure followed is analogous to that recommended and followed by Socrates in the Republic (368D): confronted by two apparently similar messages, one hard and one easy to read, we should read the easy one first and then look to see if the hard one says the same.
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