Reductionism is a method of understanding in science and philosophy that involves breaking down complex systems into their simpler parts and observing and understanding the interactions of those parts. This style of thinking and understanding can be applied to many different things, from physical objects to theories to definitions and meanings. Though the idea of reductionism has existed since the ancient Greeks, René Descartes, a 17th century French philosopher and scientist, was the first to formally state the concept. He stated that the world was like a machine composed of many smaller parts, and that it could be understood by taking apart and studying the parts before learning how they all fit into the whole.
Theoretical reductionism is one several different forms of Descartes's methods of thought. It states that all theories in a field are part of a larger theory with a broader scope. In theory, this supports the idea of the existence of a "grand unified theory" of physics that combines quantum physics with other observed phenomena.
Another form is methodological reductionism, which states that the best way to solve a problem or understand an explanation is to break it down to the smallest possible understandable parts. By this explanation, it is better to view a phenomenon such as melting or sublimation from the view of atomic interactions than from the view of the simple chemical explanations involving heat and pressure. simply speaking, by this view it is preferable to view anything from its lowest, simplest form than to look at higher level, more complex systems and explanations.
The final type is ontological reductionism, which, because of its metaphysical nature, is more useful in philosophy than in science. This theory states that reality itself is made up of a finite number of different kinds of entities, objects, or substances. Some even go so far as to say that everything that exists can be broken down to different combinations of the same single kind of substance.
Contrary to reductionism, systems thinking is a style of thought and reasoning that seeks to understand a whole system by examining the system as a whole instead of by disassembling and studying the parts. While some people prefer to use one of the two styles of thinking to the exclusion of the other, it is more common to use whichever style fits a given situation. Quite simply, some situations call for systems thinking, while others require a closer look at the parts of the system are are better suited for reductionism.