Reductionism is a philosophy that provides some interesting thought exercises. When applied to science, the basic theory is that all science can be revealed as some subset of some other part of science, therefore reducing the subset into the other discipline. In short, systems are no more than the sum of their parts. Followed to its logical conclusion, we would eventually discover that there is, in fact, only a single branch of science that could explain everything.
By way of a small example (that doesn’t work fully), we can take Kepler’s laws of planetary motion. Kepler explained how planets move. A few years later, Newton came up with his laws of dynamics which explain how everything moves. Newton’s laws explained how the planets moved as well as everything else, therefore Kepler’s work was “reduced” to become part of the general understanding of how things move.
This is understandable and even logical. As our knowledge of the natural world expands, we frequently find that previous work, usually smaller in scope, fits into a larger picture. Incidentally, this is the hallmark of a good theory; good theories allow predictions.
Let’s expand it a bit. How far can the discipline of biology be reduced? Living things are made of cells, which are made of molecules, which are made of atoms which are made of particles. Following the reductionist point of view, biology can therefore be reduced to be a branch of physics. That’s much harder to wrap your head around. Fortunately, there is another philosophy that we can use in a scientific framework to help us understand why this is so hard to grasp.
Emergence is the philosophy that some systems exhibit behaviours that cannot be explained by any of its parts. In short, the sum is greater than its parts. When applying these philosophies to science, the criteria for valid reductionism is that it is a two-way street. Meaning that in order to be considered a valid reduction, the sum does not exhibit emergent traits and the parts are all accounted for. Taking the Kepler/Newtonian example above; this works. All of Kepler’s laws are accounted for in Newton’s (reduction) and Newton’s “sum” does not behave in ways that cannot be predicted by its component parts (emergence).
However, when applying the emergence criteria to biology it seems that the sum has a great many behaviours and characteristics that cannot be explained by the parts. Since our understanding of particle physics does not provide us with a way to understand why Jimmy likes chocolate ice cream and Jenny does not, we cannot reduce human biology to particle physics. Somewhere in there is the religious argument for creation, but it will more likely turn out to be either ignorance on our part about physics, or invalidation of the emergence/reduction philosophies as scientifically useful. For the moment, however, they provide interesting ‘what if’ scenarios and playful theorizing is a valid way to pursue science and uncover new ideas.