Theories other than the most prevalent substantive theories are also discussed. Model theory studies the models of various formal theories. Theorists invoke these ideas in hopes of solving particular problems with existing theories. Others argue that he did have his own theories and beliefs. There are different ways to set up the notion of number in metaphysics theories. Facts are central to building scientific theories. They are relevant in research on unified theories such as quantum gravity. Marginalism refers to the study of marginal theories and studies within economics. Feminist literary criticism is literary criticism informed by feminist theories or politics. Belief in such theories has decreased. HAARP is the subject of numerous conspiracy theories. He radically modified classical political economic theories. There is no evidence to support any of these theories. There are a number of proposed quantum gravity theories. A number of other conflicting theories exist.