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.