Reductionism in philosophy describes a number of related, contentious theories that hold, very roughly, that the nature of complex things can always be reduced to (explained by) simpler or more fundamental things. This is said of objects, phenomena, explanations, theories, and meanings. The term is often used to criticize an imagined position rather than to describe a real one.

• Ontological reductionism is the idea that everything that exists is made from a small number of basic substances that behave in regular ways. Compare to monism.
• Methodological reductionism is the idea that explanations of things, such as scientific explanations, ought to be continually reduced to the very simplest entities possible.
• Theoretical reductionism is the idea that older theories or explanations are not generally replaced outright by new ones, but that new theories are refinements or reductions of the old theory in greater detail.
• Scientific reductionism has been used to describe all of the above ideas as they relate to science, but is most often used to describe the idea that all phenomena can be reduced to scientific explanations.
• Linguistic reductionism is the idea that everything can be described in a language with a limited number of core concepts, and combinations of those concepts. (See Basic English and the constructed language Toki Pona).
• The term "greedy reductionism" was coined by Daniel Dennett to condemn those forms of reductionism that try to explain too much with too little.