Joseph Raz is legal, moral and political philosopher. Raz's first book, "The Concept of a Legal System", was based on his doctoral thesis. A later book, "The Morality of Freedom", develops a conception of perfectionist liberalism. Raz has argued for a distinctive understanding of legal commands as exclusionary reasons for action and for the "service conception" of authority, according to which those subject to an authority "can benefit by its decisions only if they can establish their existence and content in ways which do not depend on raising the very same issues which the authority is there to settle." This, in turn, supports Raz's argument for legal positivism, in particular "the sources thesis," "the idea that an adequate test for the existence and content of law must be based only on social facts, and not on moral arguments.". Raz is acknowledged by his contemporaries as being one of the most important living legal philosophers. He has authored and edited eleven books to date, namely "The Concept of a Legal System", "Practical Reason and Norms", "The Authority of Law", "The Morality of Freedom", "Authority", "Ethics in the Public Domain", "Engaging Reason", "Value, Respect and Attachment", "The Practice of Value", "Between Authority and Interpretation", and "From Normativity to Responsibility". In moral theory, Raz defends value pluralism and the idea that various values are incommensurable.