From a philosophical point of view, the theory of forms is the central topic in Plato’s dialogues; however, there is only one single direct argument for the existence of forms. The paper deals with the logical form of this argument in Timaeus 51d3-52a7, and it corrects and clarifies some points made in a former paper on the same topic (Ferber, Gymnasium 105, 1998, 419-444). It is shown that the argument is logically valid; it is a modus ponendo ponens. The two premises of the argument are true (and the argument is therefore sound) if one accepts a Platonic distinction between knowledge (Greek “nous” in the sense of “episteme”) and true belief. The paper also presents an alternative Aristotelian reading of the premises.