Reductionism is an approach to understanding the nature of complex things by reducing them to the interactions of their parts, or to simpler or more fundamental things. It can also be described as the philosophical position that a complex system is nothing but the sum of its parts, and that an account of it can be reduced to accounts of individual constituents. Thus, it is the belief that everything that exists is made from a small number of basic substances that behave in regular ways, and is therefore in some respects comparable to Atomism.

By contrast, opposing views are represented by:

• Holism, which claims that complex systems are inherently irreducible, and more than the sum of their parts, and that a holistic approach is needed to understand them.
• Emergentism (or Emergence), which claims that complex systems and patterns arise (emerge) out of a multiplicity of relatively simple interactions.
• Vitalism, which claims that the processes of life are not explicable by the laws of physics and chemistry alone and that life is in some part self-determining and due to a vital principle distinct from physico-chemical forces. The concept of "élan vital" was coined by French philosopher Henri Bergson (1859 - 1941) in his 1907 book "Creative Evolution", and he set it in stark opposition to inert matter in the dynamic dualist philosophy he developed in the early 20th Century as a rejection of the then dominant idea that science was the main source of knowledge.

The idea of Reductionism was first introduced by Descartes in Part V of his "Discourses" of 1637, where he argued the world was like a machine, its pieces like clockwork mechanisms, and that the machine could be understood by taking its pieces apart, studying them, and then putting them back together to see the larger picture.

Reductionist thinking and methods are the basis for many of the well-developed areas of modern science, including much of physics, chemistry and cell biology. However, while it is commonly accepted that, for example, most aspects of chemistry are based on physics, it is less clear that sociology or economics is based on psychology, as some reductionists would claim.

In linguistics, Reductionism is the idea that everything can be described in a language with a limited number of core concepts, and combinations of those concepts.

• Token Ontological Reductionism is the idea that every item that exists is a sum item, and that every perceivable item is a sum of items at a lower level of complexity. Token ontological reduction of biological things to chemical things, for example, is generally accepted.

• Type Ontological Reductionism is the idea that every type of item is a sum type of item, and that every perceivable type of item is a sum of types of items at a lower level of complexity. Type ontological reduction of biological things to chemical things, for example, is often rejected.

• Epistemological Reductionism is the theory that a complex system can be explained by reduction to its fundamental parts. It holds that all phenomena can be completely understood in terms of the behaviour of micro-physical entities.