The great philosopher Aristotle famously put “The whole is more than the sum of its parts.” With this, he proposed an idea of holism. The term itself did not appear until 1926 when South African statesman Jan Smuts defined it as “The tendency in nature to form wholes that are greater than the sum of the parts through creative evolution.” in his book Holism and Evolution. Nonetheless, it is important to note that more than two thousand years back, Aristotle had this fascinating idea that instead of parts of a substance defining the substance, it is the substance that defines its parts.
Reductionism, on the other had, takes the apposite approach to explaining anything. It says that a complex system is nothing but the sum of all of its parts and understanding those parts can tell us everything about the complex system that they belong to. This idea was supported by philosophers that came out of Miletus. This includes Thales, arguably the first known philosopher of the western civilization.