In coming to grips with what are essentially logical and conceptual problems Plato doubtless had in mind and followed the example set by the natural philosophers. It seems very likely, therefore, that he thought of his Forms as causes of certain puzzling facts in a manner initially not wholly unlike that in which others had believed air or fire to be causes of various physical phenomena. Plato's language suggests that his own explanatory formula, though strikingly different in function from any other, was fashioned after those of his precursors.
Stough, Charlotte, "Forms and Explanation in the Phaedo" (1973). The Society for Ancient Greek Philosophy Newsletter. 89.