Occam's razor is the meta-theoretical principle that "entities must not be multiplied beyond necessity" (entia non sunt multiplicanda praeter necessitatem) and the conclusion thereof, that the simplest solution is usually the correct one.
Wikipedia, Occam's Razor
Occam’s razor is often invoked against the existence of God. The argument goes that if we can explain the universe without resorting to the supernatural, then it is preferable to do so. Whether or not this is a legitimate use of the razor, the argument has its appeal: why argue for the existence of something that we don’t need to explain what we see? Yet the appeal of the argument is grounded in the very fact which will undermine it.
We might rephrase the conclusion of Occam’s Razor to say that “the most elegant solution is usually the correct one”. Or, “the most beautiful solution is usually the correct one”. But really, the reference to correctness is a red herring. If the simplest theory were “usually correct”, we would have long ago exhausted all scientific knowledge. In fact no meta-theoretical principle at all will give us a result that is “usually correct”. As far as the scientific method goes, “correct” is always tentative, and the only “usual” thing about scientific theories is their being constantly overturned.
The formulation might be salvaged by saying the simplest theory is usually closer to the truth than the alternative, but the razor is more coherent closer to its original formulation: entities should not be multiplied beyond necessity. The most elegant solution is not necessarily “usually correct”, but should be preferred. On its face it’s a normative statement, but in reality it’s a bridge across the is/ought gap: elegance is an acceptable heuristic because the universe is fundamentally beautiful.
Why would this be the case? Given two theories which explain all the evidence, what gives us license to prefer the simpler, more elegant, more beautiful one? Evidence can’t help us here; this is a question of the a priori – the framework by which we interpret evidence (hence meta-theoretical in the definition). And if we say that the universe is systematically beautiful in any meaningful way, as Occam’s razor does, then we must explain that in a way that transcends chance and probability, for beauty in the sense of the elegance of an explicans is the antithesis of purposelessness.
Ultimately, the scientific process – the realm of the a posteriori – can only answer the question “how”. Yes, things can have purpose in their own narrowly defined way – for example in natural selection and adaptation, there is a sense in which a feature of an animal can be said to be “for” something: legs for locomotion, eyes for vision. But the “why” of an anatomical feature stops at its usefulness to the environment. There can be no “why”, no beauty, beyond the narrow scope of what we stipulate arbitrarily as “purpose” – in that case, survival. Yet we nevertheless speak of the beauty of nature as revealed through science; of the elegance of a theory – precisely because we have a priori heuristics like Occam’s Razor by which we can make aesthetic judgements on the “why”s among which we would have no other way of deciding.
So if we are to move beyond narrow and arbitrary “why”s into a fundamental “why”, we must make explicit the assumption behind Occam’s Razor. The search for a grand and unified physical theory of everything – the quest to unify all the fundamental forces – would prove to be the ultimate (scientific) expression of universal beauty, as well as the ultimate application of Occam’s Razor. But such a search can only be warranted if we have a reason to assume the universe is fundamentally beautiful.
Of course, Occam’s Razor is only a heuristic. It does not prove the existence of a God; it assumes it. Therefore it might be phrased: so far as Occam’s Razor is a legitimate heuristic, it suggests the existence of fundamental universal beauty – i.e., God.