Logic is purely formal—it deals only with the form of thought itself, not with any particular objects. Physics and ethics, on the other hand, deal with particular objects: physics is concerned with the laws of nature, ethics with the laws of freedom. Additionally, logic is an "a priori" discipline, i.e., logical truths do not depend on any particular experience for their justification. By contrast, physics and ethics are mixed disciplines, containing empirical and non-empirical parts. The empirical part of physics deals with contingently true phenomena, like what kind of physical entities there are and the relations in which they stand; the non-empirical part deals with fundamental concepts like space, time, and matter. Similarly, ethics contains an empirical part, which deals with the question of what—given the contingencies of human nature—tends to promote human welfare, and a non-empirical part, which is concerned with an "a priori" investigation into the nature and substance of morality.