Why should we consider activities as diverse as an amoeba dividing, a saffron crocus blooming, a tiger stalking its prey, and a human contemplating the heavens to be manifestations of a single thing, namely, life? According to Aristotle, to answer this question one must first understand the soul. The soul is life's principle-the primary cause of the ways living beings feed, reproduce, move, perceive, and think. This book starts from the premise that we can only grasp Aristotle's broadly metaphysical theses by studying how he employs them in his scientific investigations. Christopher Frey engages with a wide selection of Aristotle's texts including detailed discussions of his physical and biological works, and brings contemporary discussions of the philosophy of life into conversation with Aristotle's account, shedding light on the centrality of life to Aristotle's philosophy and on the role life should play in our endeavors to understand these questions.