William of Ockham was a 14th-century philosopher and theologian, educated at Oxford and having completed the requirements for a master’s degree but not granted such a degree. Technically a student but admitted into the ranks of the faculty at Oxford, Ockham’s commentary on Peter Lombard’s Sentences (a standard reference of Christian theology at the time) was decried as heretical by the clergy. Ockham was later excommunicated from the Church after fleeing Avignon — the location of his heresy trial — and taking asylum in the court of the Holy Roman Emperor, writing treatises justifying the emperor’s control of the church in his dominion.
Ockham’s writings were ahead of their time and reverberated centuries later as others developed science, formal logic, and computing. His famous Law of Parsimony (which we call a “razor” because it is used to cut away unnecessary complication) only pertained to theology in his own writing, but the principle of economy inspired writers from other disciplines in future generations. It was interpreted later by Bertrand Russell as stating that, if a phenomenon can be explained without including a hypothetical entity, it should be explained without it. There are untold ways of restating the principle, but in modern times, we usually state is as, “Given two theories which make the same predictions, the one with fewer assumptions should be preferred.”
One of the most well known applications of Occam’s Razor is special relativity, which was initially predicted to be the result of an ether which pervaded the universe and caused contraction of objects and slowing of time at very high velocities. Einstein’s version of special relativity satisfied the Lorentz equations but did so without proposing an undetectable ether, instead claiming that spacetime itself contracted. This made the idea of an ether superfluous to explaining the phenomenon, and so it is no longer considered.
Stronger forms of this principle than Ockham intended also exist, e.g. “The simplest explanation for some phenomenon is more likely to be accurate than more complicated explanations,” but it’s important to note that, if different predictions are made by the competing theories and they explain the same phenomenon with known data, the simpler one is preferable BUT both should be tested to see which one’s predictions best align with the new information.