Hobbes argued that everything in the world, including our thoughts and states of mind, can in principle be explained in terms of one thing and one thing only: matter in movement. Thoughts, for example, can be explained, he argued, as matter in motion in the brain. A perception of the world arises in a person's brain when movement in the external world causes movement in the brain, which is then experienced as an external object, and so on. If everything can be explained as matter in motion, including the mental, then there is no good reason to suppose that mind and matter are two fundamentally different kinds of things. Rather, it is more reasonable to assume that if everything can be explained in material terms, then everything is material. So argued