This week we learn about a famous but often misunderstood principle in philosophy, and how we can use this principle to help guide us in choosing appropriate models and theories.

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Occam refers to William of Ockham, a 14th century English logician and theologian. A “razor” in philosophy is a guiding principle that helps philosophers “shave away” unlikely or unuseful explanations for a phenomenon. In the case of Occam’s razor, overly complicated explanations are shaved away. More directly, if two competing explanations, theories, or models explain the same phenomenon equally well, philosophers generally prefer the explanation that makes the fewest assumptions. William of Ockham was not the first to propose such a principle, as it was famously used by Aristotle, St. Thomas Aquinas, and other philosophers before him. He did, however, use the principle in much of his philosophical work, and thus the razor became associated with him.

It is important to note that Occam’s razor does not make any claim about which of two competing theories is actually true, or which will continue to explain the phenomenon as new data is uncovered. Rather, it is a rule of thumb that guides scientists and philosophers when developing and choosing between models and theories. It is particularly effective against the tendency to add assumptions (called ad hoc hypotheses) to an older theory to make it fit with new conflicting evidence.

A good example of Occam’s razor at work concerns the debate between the geocentric and heliocentric models of the solar system. The goal of both models was to explain the motion of the planets as seen from earth. The geocentric model began with each planet following a simple orbit around the earth. However, it was later observed that the inner planets did not simply move across the sky but danced with the sun in a complicated pattern. The outer planets even appeared to turn around and move backward for a brief period in their orbit, before continuing through the sky. Furthermore, the planets moved at different speeds throughout their path. The geocentric model used ad hoc hypotheses to account for these new observations, assuming that the planets orbited on epicycles which themselves orbited the earth, or rather, a point very close to the earth, and that their speed was constant about a third point slightly farther from the earth. When still more discrepancies were observed, more epicycles were added until the model was thoroughly over-complicated. Meanwhile, the heliocentric model more elegantly explained all the observed phenomena by assuming all planets move in elliptical orbits around the sun. Occam’s razor supports this model, which continues to explain newly observed stellar phenomena even today.

There are many other instances in which Occam’s razor supported a successful scientific theory. For example, quantum mechanics and special relativity were both huge departures from previous theories in an effort to avoid extraneous or ad hoc hypotheses. Even before the empirical success of Occam’s razor, the idea drew support due to its aesthetic appeal. St. Thomas Aquinas, Newton, and Galileo all remarked that simplicity was inherently natural and desirable, and physicist Paul Dirac believed that theorists should strive for the mathematical beauty that often accompanies simplicity. Scientists and philosophers alike agree that all theories must be understandable and testable to be successful, therefore pragmatically speaking a model with fewer assumptions and complications is easier to work with. While opponents of Occam’s razor argue that it is subjective and stifles creativity, its successes have earned it a place as a cornerstone of modern philosophy and scientific thinking.

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