If you wake up in the morning to find your lawn wet, you are probably more likely to attribute the wetness to rain or dew than to a giant sentient ice cube with legs that stomped through your neighborhood, leaving a trail of water in its wake. Although seemingly intuitive, our appreciation for simple explanations over unnecessarily complicated ones is actually a philosophical principle called Occam’s razor. Attributed to medieval Franciscan theologian and philosopher William of Ockham, Occam’s razor is a principle commonly deployed in a wide range of disciplines, from interpretations of religious texts to string theory in physics. In general, the principle states that a simple theory—when everything else seems equal—is better than a more complicated one. While the principle seems straightforward, its validity is highly contentious.
Philosophers usually conceive of Occam’s razor in terms of two kinds of simplicity: syntactic and ontological. Syntactic simplicity refers to the elegance of a theory, meaning that the theory itself is concise, relying on fewer assumptions than other theories. By contrast, ontological parsimony refers to the object a theory is trying to explain, specifically the object’s simplicity as a phenomenon. In debates surrounding the philosophy of mind, Occam’s razor is often cited in defense of physicalism—the concept that everything, including our mental state, can be reduced to physical things or processes or their properties. In contrast to physicalism, dualismpostulates that reality consists of two distinct elements, mind and matter. Physicalism can be seen as an example of ontological parsimony, because the object it describes—physical existence—requires a singular entity, as opposed to the two entities required by dualism. However, physicalism can also be interpreted as more complex, and hence less elegant, than dualism, because it requires us to conceptualize what seem to be two basic kinds of entities as ultimately being one kind. In terms of syntactic simplicity, then, dualism can be seen as the more straightforward concept. Because of Occam’s razor’s ability to justify multiple competing theories, some critics believe that the principle is too interpretation-based to be useful.
One of the fields known for significantly relying on Occam’s razor is theoretical physics. Some of physics’ most iconic scientists have used the principle, including Galileo Galilei, who argued that the relative simplicity of a heliocentric model of the universe made that model more plausible than Ptolemy’s geocentric model. In modern physics, ether theories, which proposed that all matter and space is filled with an invisible, undetectable medium through which electromagnetic waves can travel, were abandoned in favor of the theory of special relativity, which requires no such medium and hence seems simpler. But simplicity does not make a theory unconditionally true; it can be interpreted in many different ways, and it can often be used to argue in favor of older, discredited theories.
The search for a “theory of everything,” a theory that can explain and predict every physical phenomenon without contradiction, can be interpreted as another instance in which scientists have used Occam’s razor to develop elegant explanations of the natural world. However, many believe this pursuit to be misguided. Critics of Occam’s razor in physics cite the impossibility of systematizing and simplifying all natural phenomena; they also point to the risk of sacrificing accuracy when simplicity is prioritized. A joke among theoretical physicists is the spherical cow: physicists are so eager to simplify things, goes the joke, that, in many equations, the physical dimensions of a cow become equal to that of a sphere.
When used incorrectly, Occam’s razor can have more serious consequences. In medicine, the maxim “When you hear hooves, think horses, not zebras” is taught to practitioners to remind them that a simpler diagnosis that can explain multiple symptoms is more probable than a series of unconnected and rare conditions. However, misdiagnoses can happen if practitioners apply only the criterion of simplicity when analyzing symptoms.
Even though William of Ockham might have used his own principle so sharply that it became known as a razor, modern academics and professionals are more hesitant to apply the criterion of simplicity liberally to all reasoning. Because it can lack firmness and consistency when applied to complex ideas or phenomena, Occam’s razor is more commonly seen as a guiding heuristic than as a principle of absolute truth.