This book is an inquiry into an important yet neglected area of Aristotle’s philosophy: the generation of substances. All change for Aristotle is a form of coming-to-be (or passing-away) in some sense. With some changes the substance already exists and simply becomes qualified in some way, as for example when Socrates (a substance) comes to be healthy from being unhealthy (a change in quality) or comes to be bigger from being smaller (a change in quantity). Aristotle calls this coming-to-be in the qualified sense (gignesthai ti). In cases of substantial generation (what Aristotle calls coming-to-be in the unqualified sense, gignesthai haplôs) the change results in a new substance coming into existence. At the end of development, we say that Socrates has come to be (full stop) rather than saying he has come to be F. This distinction was not universally accepted by Aristotle’s predecessors.