Monism, because it describes a belief in one substance, can be used in two distinct ways:

• To describe the view that only matter, or the physical body, exist.
• To describe the view that only mind, or spirit, exist.

The first option is called "materialism" or "physicalism"; the second option is called "idealism" (which you may remember from our discussion of Berkeley in TOK).

Whilst few people accept the idealist viewpoint, materialism is quite a widely held view. The main task for the materialist, however, is to show that consciousness and the mind can be accounted for in terms of properties of matter and the functioning of a physical substance: the brain.