Long before Aristotle wrote Metaphysics, one of the issues stirring heated debate among philosophers was the question of substance. The philosophical notion of substance can be difficult to grasp as it has many nuances, but its core meaning remains relatively uncomplicated. Plato's metaphysics holds a very significant place in the history of philosophy in its own right. But it also provides an essential key to understanding Aristotle's position. Aristotle criticizes Plato's ideas about substance throughout Metaphysics, and especially in books Zeta, Mu, and Nu. Aristotle's critique concentrates on Plato's idea that forms are both incorporeal (bodiless), eternal, independently existing entities and the causes of existence. Substance is "form present in matter", but not a compound of form and matter. Aristotle rejects the latter option, because compounds cannot be primary. This philosophically rigorous and innovative idea made Aristotle one of the most important metaphysicians in the history of philosophy.