This chapter discusses the two main tracks in thinking about the human mind that still influence our conceptions today. These main tracks can be called the mind as consciousness and the mind as rationality. The first track focuses on the mind primarily as a mechanism for experiencing the world, while the second track regards the mind as a designation for our ability to think and act rationally. These two tracks are still both very important in current psychological research: the consciousness standpoint underlies much of neuropsychology, where consciousness is often sought to be reduced to something neurological, whereas the rationality standpoint primarily underlies various forms of cultural psychology. Many modern consciousness researchers regard themselves as materialists and therefore cannot accept Descartes' idea of two essentially different metaphysical substances, only one of which is material. For Aristotle, substance is something that has the possibility of changing, while form is what makes a particular substance what it is.