Monismus

Monismus was coined by Christian Wolff in 1720, from Greek monos (alone, single). Wolff created it as the opposite of his other coinage, 'dualism.' Where dualists see two fundamental substances — mind and body — monists see one. The word was a philosophical tool: a label for positions that insist on unity rather than division.

Baruch Spinoza is the most famous monist in Western philosophy, though he wrote before the word existed. His Ethics (1677) argued that there is only one substance — God or Nature (Deus sive Natura). Mind and body are not two substances but two attributes of the one substance. The Jewish community of Amsterdam had excommunicated Spinoza in 1656 with one of the harshest cherem (bans) ever issued. His books were banned. His monism was heresy because it identified God with the natural world.

In the nineteenth century, Ernst Haeckel popularized monism in Germany as a scientific worldview. His Monist League (Monistenbund), founded in 1906, promoted monism as an alternative to religious dualism. Haeckel's version was explicitly materialist — one substance, and that substance is matter-energy. This was monism as ideology, not just metaphysics.

Modern philosophy of mind has revived monism in new forms. Physicalism — the view that everything is physical — is a form of materialism. Neutral monism, associated with William James and Bertrand Russell, holds that mind and matter are both made of a more fundamental stuff that is neither mental nor physical. The word Wolff coined in 1720 still names the live options in contemporary philosophy. One thing. The argument is about which thing.