Alfred North Whitehead was a mathematician who had contributed to the foundations of logic in Principia Mathematica, written with Russell. In the 1920s, in his sixties, he moved to Harvard and set about something quite different: a new metaphysics that took account of the changes in physics — relativity and quantum theory.

His starting point is this: Western thought builds reality out of substances enduring through time (matter, body, subject) and treats change as something that happens to them. Whitehead reverses it. The ultimate units of reality are not things but events: momentary processes he calls actual entities, which come to be, complete themselves, and then become data for what follows. Each event 'prehends' the events before it and makes a whole out of them; the objects that appear to endure are repeating patterns in these series of events. A stone is a long-lived pattern of process.

Process philosophy remained marginal in his own field — the analytic philosophy of the 1930s was suspicious of metaphysics, and the book's language is heavy. But it took hold elsewhere: in process theology, in ecological thought (for seeing nature as a web of relations rather than a stock of objects), in philosophy of biology and, in recent years, in debates about consciousness. Deleuze cited him openly; his trace is in the science studies of Latour and Stengers. Whitehead is a good example of a position in the history of philosophy that lost without being exhausted: his questions never left the field's agenda, while the language of his answers remains difficult.