Jean Perrin’s argument for the existence of molecules from his 1908 experimental determination of Avogadro’s number raises two questions considered in this article. One is historical: Why as late as 1908 should Perrin have thought it necessary to argue that molecules exist? The other, which takes up the bulk of this article, is philosophical: In view of the fact that his argument appears to assume the existence of molecules as a premise, how, if at all, can a charge of circularity be avoided? I criticize attempted reconstructions of his argument by Salmon, Glymour, and hypothetico-deductivists. And I propose a probabilistic solution that avoids circularity. Finally, I consider whether Perrin’s reasoning, so construed, is an empirical argument for scientific realism.

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