Holism (from holos, a Greek word meaning all, entire, total) is the idea that all the properties of a given system (biological, chemical, social, economic, mental, linguistic, etc.) cannot be determined or explained by the sum of its component parts alone. Instead, the system as a whole determines in an important way how the parts behave.

The general principle of holism was concisely summarized by Aristotle in the Metaphysics: "The whole is more than the sum of its parts".

Reductionism is seen as the opposite of holism. Reductionism in science says that a complex system can be explained by reduction to its fundamental parts. Essentially, chemistry is reducible to physics, biology is reducible to chemistry and physics, and psychology and sociology are reducible to biology, etc.