  Aristotle's account of Plato's reasons for introducing Forms indicates that change and essence are critical to Plato's thinking about the deficiency of material particulars. “…Socrates sought the universal in them and turned to definitions. Plato, accepting this, thought that this (defining) comes to be about different things, and not about sensibles. For it is impossible that the common definition be about any of the sensibles, for these are always changing.” (Metaphysics 987b1–7) At the very beginning, then, the search for knowledge leads to definitions. The question is where one can find definitions or definables. Aristotle asserts that Plato thought that definitions could not be found in the sensibles because they were always changing.