Marjorie Grene has argued in a number of contexts (1971, 1974, 1976) that any attempt to reduce the explanation of human actions to “mechanistic” explanations is doomed to failure in advance because it is absurd. “This argument examines the status of the reductivist thesis in its own terms and reduces it to absurdity …” (1971, p. 55); “the attempt to reduce human purposive, or ‘intentional,‘ action to physiology and ultimately to physics and chemistry is an absurdity rather than simply a confusion” (1976, p. 183). I wish to show that this argument is fallacious because it involves a petitio principii; and if Plato can be credited with introducing the same argument in the Theaetetus, as she suggests, so much the worse for Plato.