The three Telluride Lectures contained in this booklet were delivered at Cornell University on November 11, 12 and 13 1957. They are here printed precisely as they were given. They were designed to deal with Marxism as a theory or as a social and political philosophy but not to present it either historically, as it originated a century ago in the writings of Marx and Engels and as it was modified by Lenin, or systematically as a body of more or less coherent philosophical propositions.