This essay is an expanded version of an essay originally written for the Harper Guide to Philosophy, edited by Arthur Danto. For the second edition I have kept the original text, and appended six additional essays. I have made changes in and additions to the bibliography. In writing the original text I was deeply indebted for advice and encouragement to Arthur Danto, Michael Podro, Adrian Stokes, Bernard Williams, and Margaret Cohen. In preparing this new edition I have benefited a great deal from criticism, suggestion, and assistance from David Carrier, Richard Dammann, Hidé Ishiguro, Jerrold Levinson, Charles Rosen, David Wiggins, Bruno Wollheim, and Henri Zerner: I have a singular debt of gratitude to Antonia Phillips. Two of the additional essays derive from a symposium held during the fifth Bristol Philosophy Conference in 1976 with Nelson Goodman and David Wiggins, from whose comments I learnt. I owe a very great deal to Katherine Backhouse who has typed and retyped the manuscript for both editions. I am grateful to Jeremy Mynott and Jonathan Sinclair-Wilson for editorial advice and assistance.