his book is a venture in the metaphysics of science, the exploration of the most basicfeatures of the world implied or presupposed by science. One of its main aims is todemonstrate the fundamental importance of such an investigation. Getting this verygeneral picture right makes a real difference to whether we do the science well andunderstand properly what it tells us. The particular metaphysical thesis that motiv-ates this book is that the world—at least insofar as living beings are concerned—ismade up not of substantial particles or things, as philosophers have overwhelminglysupposed, but of processes. It is dynamic through and through. This thesis, webelieve, has profound consequences.More specifically, we propose that the living world is a hierarchy of processes,stabilized and actively maintained at different timescales. We can think of thishierarchy in broadly mereological terms: molecules, cells, organs, organisms, popu-lations, and so on.