  Thus far we have seen that Leibniz rejected the Cartesian account of matter, according to which matter, the essence of which is extension, could be considered a substance. Leibniz held instead that only beings endowed with true unity and capable of action can count as substances. The ultimate expression of Leibniz's view comes in his celebrated theory of monads, in which the only beings that will count as genuine substances and hence be considered real are mind-like simple substances endowed with perception and appetite. What was said above concerning the unity and activity of simple substance should suffice to explain Leibniz's reasons for holding such a position. Now a fuller version of Leibniz's idealism must be presented.