There may be no end to the number of philosophies a human mind can build. Not empty ones, not frauds, but coherent, honest systems raised on real distinctions and carried through to the last decimal of clarity. Coherence, it turns out, is cheap. Yet out of this endless field of possible worlds, only a handful live for thousands of years. One of them bears the name of a quiet Greek of the third century who was so reluctant to draw attention to himself that we do not know the year or the place of his birth.

Plotinus built the last great complete edifice of ancient thought, and he was convinced he had invented nothing at all. He believed he was merely reading Plato correctly. Under the modest cover of a commentary he produced a system that became, for seventeen centuries, the language in which East and West spoke about the highest things. Augustine thought in its categories. Islamic and Jewish philosophers argued inside its frame. The Florentine Renaissance rediscovered the world through it. And when Kant drew the border of human reason, and when phenomenology learned to speak of pure consciousness, they were pushing off—often without knowing it—from a move first made in the Roman schoolroom of a poor and unworldly teacher.

This book follows that single thread from its source—an unanswered question Plato left lying in the road—through the life and writings of Plotinus himself, through his heirs and his enemies, through the long millennium in which his system was simply the air, and onward to Descartes, Spinoza, Leibniz, Kant, phenomenology, and at last to what modern brain science says about perception and to the machines that have lately learned to think. Along the way it asks of these texts not only the historian's question, what did they claim, but a second, quieter one that keeps old philosophy alive: what were these people doing in the very moment they made their claim?