  One possibility (see Gill 1996, 22) is that Plato is alluding to the middle period thesis that only certain types of properties summon the understanding to think about forms. For example, in the Republic, Socrates claims that whereas “the soul isn’t compelled to ask the understanding what a finger is, since sight doesn’t suggest to it that a finger is at the same time the opposite of a finger” (523d2–6), the soul is compelled to ask what the large is and what the small is when sight suggests to it that the index finger is both big (relative to the pinky) and small (relative to the middle finger). If forms were merely posited to explain the compresence of contrary properties in sensible things, then there would be no need to posit a form corresponding to properties (such as water and dirt) that have no contraries. However, this is unlikely to be the source of Socrates’ worry here, for the Republic passage does not discuss metaphysical reasons for positing the existence of forms, but rather discusses the psychological and epistemic question of what prompts the soul to think of forms that have already been posited.