Monists are philosophers who believe that all of reality, everything that exists, falls into one and only one basic category of being. There is, according to every form of monism, only one fundamental sort of substance in existence. Everything is therefore somehow a configuration of this one substance. In contemporary physics, string theory can be thought of as just the most recent version of monism, proclaiming as it does that everything is ultimately composed of one-dimensional strings of energy — whatever exactly that means.

Historically, two main forms of monism exist: Materialism and idealism.

• Materialism is the view that all that exists is matter, configured into material objects. On the materialist view, there are no minds or souls or immaterial spirits. Physical matter, in all its permutations and combinations, is all that exists.
• Idealism is the view that all that exists are minds (immaterial thinking things) and ideas in minds. According to the idealist, nonmental matter is an illusion projected by our minds. The entire physical universe is just bundles of ideas, a virtual reality, perhaps produced by the mind of God. Bishop George Berkeley (1685-1753) was perhaps the greatest of the idealist philosophers. There are not many idealists, in the strict philosophical sense, around nowadays, although some developments in physics have caused a few contemporary philosophers to rethink this view.