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.