 The theory of dual or double aspects is usually traced back to Spinoza (1677). The fundamental idea uniting the family of views that are gathered together under this label is that there is an underlying reality that we can grasp as mental or as physical, depending on the point of view from which we apprehend it. The mental and the physical are aspects of this underlying reality which, in itself, is neither mental nor physical. Each one of us can know their own brain under both of these aspects—via introspection and (scientific) observation. But the claim of the theory is quite general: everything there is is to be understood as consisting of an underlying reality that has these two aspects.