Occam’s razor gets its name from the medieval theologian William of Occam, who wrote in the 14th century that “No more things should be presumed to exist than are absolutely necessary.” Put another way, the simplest conclusion is usually best. That’s Occam’s razor. It means shaving off unnecessary or dubious premises and seeing what remains. It’s about being cautious—indeed, conservative—when reaching a conclusion.
A conclusion is best—most likely to be correct—when it’s encumbered by the fewest premises, and those premises should link up with the conclusion in a way that feels natural and unstrained.
For example, if you saw a light pass quickly overhead through the night sky, Occam’s razor suggests that you would be wise to attribute it to a meteor before, say, an alien spacecraft from a distant galaxy. Though the alien spacecraft theory is logically possible, it’s burdened by numerous questionable premises (such as that spacefaring aliens exist and intergalactic spaceflight is possible). By contrast, the meteor theory is simpler; it requires one key premise that is easily believed: when entering Earth’s atmosphere, meteors catch fire and people see them. It’s a common occurrence. We already know it happens. Following Occam’s razor, what was seen overhead is likely an instance of this. The meteor theory coheres well with our already well-established background knowledge (the things we take for true, and with good reasons).
But one should be cautious about adhering too slavishly to Occam’s razor. As Alfred North Whitehead, the famous 20th century philosopher who supervised Bertrand Russell’s doctoral dissertation, wrote, “Seek simplicity and distrust it.” One reason to be cautious is that Occam’s razor can ease you toward confirmation bias, by which you might attend closely to the hits with your background knowledge, but not the misses. That is, wherever your background knowledge is confirmed, you might discount or notice little else. And the simplest model of a thing or phenomenon may make for ease of thought, but we should also distrust ease (as we distrust simplicity). The easiest path is not always best. It can make for illusion. Existence is complicated and doesn’t always cooperate with our most comfortable frames.
But you already knew that.
In The Critique of Pure Reason, Emmanuel Kant formulated a counter to Occam’s razor: “The variety of beings should not rashly be diminished.” Leonard Susskind, a Stanford physicist and strong advocate of the multiverse hypothesis (a rather extravagant explanation for the way things are, if ever there was one), would no doubt agree. Occam’s razor in one hand, an open mind in the other.