Physicalism

In philosophy, physicalism is the metaphysical thesis that "everything is physical", that there is "nothing over and above" the physical, or that everything supervenes on the physical. Physicalism is a form of ontological monism—a "one substance" view of the nature of reality as opposed to a "two-substance" (dualism) or "many-substance" (pluralism) view. Both the definition of "physical" and the meaning of physicalism have been debated.

Quotes

• Physicalists believe that nothing exists but the physical world that can be studied by science: the world of objective reality. But then they have to find room somehow for feelings, desires, thoughts, and experiences—for you and me—in such a world.
  One theory offered in defense of physicalism is that the mental nature of your mental states consists in their relations to things that cause them and things they cause. For instance, when you stub your toe and feel pain, the pain is something going on in your brain. But its painfulness is not just the sum of its physical characteristics, and it is not some mysterious non-physical property either. Rather, what makes it a pain is that it is the kind of state of your brain that is usually caused by injury, and that usually causes you to yell and hop around and avoid the thing that caused the injury. And that could be a purely physical state of your brain.
  But that doesn’t seem enough to make something a pain. It’s true that pains are caused by injury, and they do make you hop and yell. But they also feel a certain way, and that seems to be something different from all their relations to causes and effects, as well as all the physical properties they may have—if they are in fact events in your brain. I myself believe that this inner aspect of pain and other conscious experiences cannot be adequately analyzed in terms of any system of causal relations to physical stimuli and behavior, however complicated.
  There seem to be two very different kinds of things going on in the world: the things that belong to physical reality, which many different people can observe from the outside, and those other things that belong to mental reality, which each of us experiences from the inside in his own case. This isn’t true only of human beings: dogs and cats and horses and birds seem to be conscious, and fish and ants and beetles probably are too. Who knows where it stops?
  We won’t have an adequate general conception of the world until we can explain how, when a lot of physical elements are put together in the right way, they form not just a functioning biological organism but a conscious being. If consciousness itself could be identified with some kind of physical state, the way would be open for a unified physical theory of mind and body, and therefore perhaps for a unified physical theory of the universe. But the reasons against a purely physical theory of consciousness are strong enough to make it seem likely that a physical theory of the whole of reality is impossible. Physical science has progressed by leaving the mind out of what it tries to explain, but there may be more to the world than can be understood by physical science.
  • Thomas Nagel, What Does It All Mean? (1987), Chap. 4 : The Mind-Body Problem