Occam’s Razor: one should not increase, beyond what is necessary, the number of entities required to explain anything. Via Wikipedia:

Occam’s razor (also written as Ockham’s razor and in Latin lex parsimoniae) is a problem-solving principle devised by William of Ockham (c. 1287–1347), who was an English Franciscan friar and scholastic philosopher and theologian. The principle states that among competing hypotheses, the one with the fewest assumptions should be selected. Other, more complicated solutions may ultimately prove correct, but—in the absence of certainty—the fewer assumptions that are made, the better.

The application of the principle can be used to shift the burden of proof in a discussion. However, Alan Baker, who suggests this in the online Stanford Encyclopedia of Philosophy, is careful to point out that his suggestion should not be taken generally, but only as it applies in a particular context, that is: philosophers who argue in opposition to metaphysical theories that involve allegedly “superfluous ontological apparatus”. Baker then notices that principles, including Occam’s Razor, are often expressed in a way that is not clear regarding which facet of “simplicity” — parsimony or elegance — is being referred to, and that in a hypothetical formulation the facets of simplicity may work in different directions: a simpler description may refer to a more complex hypothesis, and a more complex description may refer to a simpler hypothesis.

Solomonoff’s theory of inductive inference is a mathematically formalized Occam’s Razor: Shorter computable theories have more weight when calculating the probability of the next observation, using all computable theories which perfectly describe previous observations.

In science, Occam’s Razor is used as a heuristic (discovery tool) to guide scientists in the development of theoretical models rather than as an arbiter between published models. In the scientific method, Occam’s Razor is not considered an irrefutable principle of logic or a scientific result; the preference for simplicity in the scientific method is based on the falsifiability criterion. For each accepted explanation of a phenomenon, there is always an infinite number of possible and more complex alternatives, because one can always burden failing explanations with ad hoc hypothesis to prevent them from being falsified; therefore, simpler theories are preferable to more complex ones because they are better testable and falsifiable.