Inquiry regarding fallacies involving likelihood

What exactly makes a theory with less commitments more likely? How did we ever come to that conclusion, and couldn’t it be wrong? Im just a layman in this field, humor me. Thanks.

Based on you clarification, it sounds as if you are referring to the principle of parsimony, which states that the most acceptable explanation of an occurrence, phenomenon, or event is the simplest, involving the fewest entities, assumptions, or changes. 

First, it should be clear that we are talking about the most acceptable explanation, not the most likely. When we don't have access to the numbers involved to do the math, like in your brain in the vat example, we are not dealing with probability but plausibility. Bayesian statisticians might disagree, but that's a different discussion. Second, a significant part of this principle is the word "simple," which applies to the assumptions made; it is not just about the number of assumptions.

If it were just about the number of "commitments" (assumptions), then we can simply assume magic exists (one assumption) and use that as the explanation for everything. But the reason we can't is that "magic" would be far from "simple."

Back to your brain in the vat theory example. We don't have access to the numbers required to calculate the probability of such a universe where this is the case. We can make initial assumptions and use Bayesian probability and we can make reasoned arguments for and against, but in any case, "common sense" would have very little to do with this as would classic probability theory. This is more of a philosophical musing than anything else.

Do read the comment above by Rationalissimus of the Elenchus regarding the conjunction fallacy . This explains why fewer assumptions are mathematically more probable in general.