Methodological reductionism is the scientific attempt to provide an explanation in terms of ever-smaller entities. Reductionist thinking suggests that to understand how a complex system works, such as an organism, you have to break it down into its component parts—understanding how the whole works by understanding the parts in isolation.
Reductionism is a powerful way of knowing but ignores other ways of knowing. For example, when different actors in a system interact together—whether species in an environment or consumers in a marketplace—we often get 📝Emergent Properties not predictable from looking at the parts (via 📝Symbiotic Earth).