Reductionism is the view that material particles and the physical forces between them, which are supposed to be mathematically analytical, can explain all that happens in the world. Chemistry is thought to be reducible to physics, biology reducible to chemistry, psychology (via neuroscience) reducible to biology, and mind/brain (or cognitive science) reducible to psychology. All these causal relations are called "bottom-up." (Info Philosopher)

"all reality and our experience of it can be reduced to ultimate indivisible elements" - Ackoff

"belief that a complex system can be explained by reduction to its fundamental parts." - Wikipedia

"The methodological concept according to which complex objects or issues must be subdivided for their study into smaller part to make their understanding easier and the process repeated as many times as needed (proposed by Descartes)" - Francois