Alexander of Aphrodisias’s Commentary on Aristotle’s Metaphysics is an important testimony to understand Plato’s philosophy.In fact,Alexander uses some lost Aristotelian books,especially a work On the Good,from which we learn that Plato’s metaphysics is adialectical metaphysics,founded on an original opposition of two principles that shapes the whole reality—these principles being the One and the indefinite Dyad.Sensible things participate in ideas(they receive their being from ideas)and the intermediate mathematical entities lie between these two realities.However,ideas can be traced back to ideal numbers and the principles of ideal numbers are the One and the indefinite Dyad.Thus,these principles constitute their metaphysical foundation of ideas and,through the ideas,of the whole reality.