Monism holds that reality consists of a single substance or principle. It contrasts with dualism, which holds that there are two, and pluralism, which holds that there are many. Monism comes in different forms. Materialism is a monism: only matter exists. Idealism is a monism: only mind or spirit exists. Neutral monism holds that the fundamental stuff is neither mental nor physical, but something prior to both. Spinoza argued that there is only one substance, which he called God or Nature, and that mind and body are two attributes of it.

Monism has a long history. The pre-Socratic philosophers sought a single principle behind the diversity of things. Thales said water. Anaximenes said air. Heraclitus said fire. Parmenides said being. In Indian philosophy, Advaita Vedanta holds that reality is a single, non-dual consciousness. Monism is attractive because it avoids the interaction problem that plagues dualism. If there is only one kind of thing, there is no mystery about how two kinds interact. But monism faces its own problems. If everything is matter, how do we explain consciousness? If everything is mind, how do we explain the apparent independence of the physical world? The debate between monism, dualism, and pluralism continues.

Forms of monism

• Materialism. Only matter exists.
• Idealism. Only mind or spirit exists.
• Neutral monism. A single neutral substance underlies both mind and matter.
• Priority monism. The whole is prior to its parts.

Monism is not the same as simplicity. A single substance can be complex and have many attributes.