A theory that all complex systems can be completely understood in terms of their components WordNet 3.0 & Open English WordNet

Reductionism is any of several related philosophical ideas regarding the associations between phenomena which can be described in terms of other simpler or more fundamental phenomena. Wikipedia

A range of philosophical systems Wikipedia Disambiguation

Philosophical view explaining systems in terms of smaller parts Wikidata

A philosophical position which holds 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. In a reductionist framework, the phenomena that can be explained completely in terms of relations between other more fundamental phenomena, are called "epiphenomena". Wiktionary