This article examines a complex passage of Aristotle's Physics in which a Pythagorean doctrine is explained by means of a mathematical example involving gnomons. The traditional interpretation of this passage (proposed by Milhaud and Burnet) has recently been challenged by Ugaglia and Acerbi, who have proposed a new one. The aim of this article is to analyse difficulties in their account and to advance a new interpretation. All attempts at interpreting the passage so far have assumed that ‘gnomons’ should indicate ‘odd numbers’. In this article it is argued that the usage of ‘gnomon’ related to polygonal numbers, which is normally considered late, could be backdated to at least the fifth/fourth centuries b.c.; in particular, it explains the link between the philosophical explanandum and the mathematical explanans in Aristotle's passage.
