Monism

Monism is the philosophical doctrine according to which reality as a whole reduces to a single principle: one substance, cause or fundamental nature. It stands opposed to dualism, which divides reality into two irreducible substances, and to pluralism, which admits several.

The term was introduced in the eighteenth century by Christian von Wolff in his Logic (1728), to designate currents of thought that sought to eliminate the separation between body and mind and to explain phenomena through a unifying principle. The idea, however, is much older. The Presocratics already searched for the arche, the principle of all things: for Thales it was water, for Anaximenes air and for Heraclitus fire, while Parmenides described being as one, unmoving and indivisible. In India, the Vedic tradition and Advaita Vedanta likewise defended a single ultimate reality, Brahman.

Modern authors have understood monism in different ways. Spinoza held that there is a single substance, which he called God or Nature, of which we know two attributes, extension and thought. For Hegel and Schelling reality is an absolute identity. Berkeley reduced the real to perception, with his famous formula esse est percipi. On the materialist side, Democritus, d'Holbach and La Mettrie maintained that everything is matter, and Marx himself started from that basis in his dialectical materialism.

A simple example helps to clarify the difference. Asked whether a person is body and soul, the dualist will say they are two distinct realities; the monist will hold that there is only one, and that the mental and the physical are its manifestations.

Contemporary debate has shifted towards the philosophy of mind. Extreme forms, such as logical behaviourism or the identity theory, have been widely criticised, and today reductive physicalism, eliminativism, Donald Davidson's anomalous monism and functionalism coexist. Whether mental properties can be reduced to the physical remains an open question.