The eighth book of Aristotle's Physics is the culmination of his theory of nature. He discusses not just physics, but the origins of the universe and the metaphysical foundations of cosmology and physical science. He moves from the discussion of motion in the cosmos to the identification of a single source and regulating principle of all motion, and so argues for the existence of a first `unmoved mover'. Daniel Graham offers a clear, accurate new translation of this key text in the history of Western thought, and accompanies the translation with a careful philosophical commentary to guide the reader towards an understanding of the wealth of important and influential arguments and ideas that Aristotle puts forward.