This is a book that celebrates the power of ideas: thought can, and does, change the world. And, in turn, ideas evolve. Fundamentalism, environmentalism and bioethics are defining our future just as Marxism, feminism or existentialism have influenced our present. So what do we need to know as we move into the 21st century? More than a simple reference work, this is A.C. Grayling's personal and heartfelt guide to the ideas, past and present, that shape our world. Covering religion, philosophy, scientific theory and political movements, each alphabetically ordered entry illuminates, elucidates and provokes. Written with Grayling's customary fire and erudition, the result is a book that aims both to arm readers with knowledge and engage them in philosophical debate.