This book examines Aristotle’s four causes (material, formal, efficient, and final), offering a systematic discussion of the relation between form and matter, causation, taxonomy, and teleology. The overall aim is to show that the four causes form a system, so that the form of a natural thing relates to its matter as the final cause of a natural process relates to its efficient cause. Aristotle’s Four Causes reaches two novel and distinctive conclusions. The first is that the formal cause or essence of a natural thing is not a property of this thing but a generic natural thing. The second is that the final cause of a process is not its purpose but the course that processes of its kind typically take.