In Ennead II.4 Plotinus investigates the question of what underlies the forms that constitute the contents of our minds and senses. Aristotle had called this substrate "matter, " and Stoic philosophers followed suit. With a critical review of their notions, and reference to Plato’s so-called Receptacle, Plotinus develops an account of matter that makes it a supremely negative entity. How he describes the indescribable, and how he justifies incorporeal matter’s indispensability to bodies, are highlights of this tenaciously argued essay.