While the Separability Thesis implies that there are no necessary moral criteria of legal validity, it leaves open the question of whether there are possible moral criteria of validity. Inclusive legal positivists (also known as soft positivists and incorporationists) believe there can be such criteria; that is, they believe there are conceptually possible legal systems in which the criteria for legal validity include (or incorporate) moral principles. Exclusive legal positivists (also known as hard positivists) deny there can be moral criteria of validity. Exclusive positivists claim the existence and content of law can always be determined by reference to social sources. This essay is an overview of the distinguishing commitments of positivism and inclusive positivism; it describes and evaluates some of the most influential criticisms of inclusive positivism.