On Theodorus' Lesson in the Theaetetus 147D-E by S
On Theodorus’ lesson in the Theaetetus 147d-e by S. Negrepontis and G. Tassopoulos In the celebrated Theaetetus 147d3-e1 passage Theodorus is giving a lesson to Theaetetus and his companion, proving certain quadratic incommensurabilities. The dominant view on this passage, expressed independently by H. Cherniss and M. Burnyeat, is that Plato has no interest to, and does not, inform the reader about Theodorus’ method of incommensurability proofs employed in his lesson, and that, therefore, there is no way to decide from the Platonic text, our sole information on the lesson, whether the method is anthyphairetic as Zeuthen, Becker, van der Waerden suggest, or not anthyphairetic as Hardy-Wright, Knorr maintain. Knorr even claims that it would be impossible for Theodorus to possess a rigorous proof of Proposition X.2 of the Elements, necessary for an anthyphairetic method, because of the reliance of the proof of this Proposition, in the Elements, on Eudoxus principle. But it is not difficult to see that Knorr’s claim is undermined by the fact that an elementary proof of Proposition X.2, in no way involving Eudoxus principle, is readily obtained either from the proof of Proposition X.3 of the Elements itself (which is contrapositive and thus logically equivalent to X.2), or from an ancient argument going back to the originator of the theory of proportion reported by Aristotle in the Topics 158a-159b passage. The main part of the paper is devoted to a discussion of the evidence for a distributive rendering of the plural ‘hai dunameis’ in the crucial statement, ’because the powers (‘hai dunameis’) were shown to be infinite in multitude’ in 147d7-8 (contrary to the Burnyeat collective, set-theoretic rendering), necessarily resulting in an anthyphairetic method for Theodorus’ proofs of incommensurability (contrary to the Cherniss-Burnyeat neutrality thesis).
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