A brief, accessible history of the idea of purpose in Western thought, from ancient Greece to the present. Can we live without the idea of purpose? Should we even try to? Kant thought we were stuck with purpose, and even Darwin's theory of natural selection, which profoundly shook the idea, was unable to kill it. Indeed, teleological explanation-what Aristotle called understanding in terms of "final causes"-seems to be making a comeback today, as both religious proponents of intelligent design and some prominent secular philosophers argue that any explanation of life without the idea of purpose is missing something essential. In On Purpose, Michael Ruse explores the history of the idea of purpose in philosophical, religious, scientific, and historical thought, from ancient Greece to the present. Accessibly written and filled with literary and other examples, the book examines "purpose" thinking in the natural and human world. It shows how three ideas about purpose have been at the heart of Western thought for more than two thousand years. In the Platonic view, purpose results from the planning of a human or divine being; in the Aristotelian, purpose stems from a tendency or principle of order in the natural world; and in the Kantian, purpose is essentially heuristic, or something to be discovered, an idea given substance by Darwin's theory of evolution through natural selection. On Purpose traces the profound and fascinating implications of these ways of thinking about purpose. Along the way, it takes up tough questions about the purpose of life and whether it's possible to have meaning without purpose, revealing that purpose is still a vital and pressing issue.