Occam's razor

The principle is named after the 14th-century Franciscan friar William of Ockham, though he did not invent it; he applied it forcefully against what he saw as needless metaphysical multiplication in the scholastic philosophy of his day. The most quoted form (“entities should not be multiplied beyond necessity”) is a Latin paraphrase that postdates Ockham, but the underlying idea is faithful to him.

The razor is a heuristic, not a law. Two hypotheses that fit the evidence equally well rarely actually fit it equally well — usually one fits the evidence better and the other only fits it after enough auxiliary assumptions are added to rescue it. The razor's real work is to penalize those auxiliary assumptions: each one is a place the hypothesis can fail, and each one is a piece of structure for which independent justification is owed.

In philosophy of religion the razor cuts both ways. Atheists invoke it against theism: if natural processes can in principle account for the universe, life, and mind, adding a deity is multiplying entities. Theists invoke it back: a single, unified divine cause may be more parsimonious than the multiverse, brute-fact constants, and abiogenesis that naturalism strings together. Both sides agree the razor matters; they disagree on which hypothesis is actually simpler when all the structure is laid out.

Modern probability theory gives the razor a precise foundation: under reasonable priors, simpler hypotheses receive higher prior probabilities, and a Bayesian who updates on evidence will end up favoring simpler explanations of the same data — the so-called Bayesian Occam's razor. This formal version makes clear that the principle is not arbitrary intellectual taste; it falls out of the mathematics of belief updating.