Ignotum per ignotius (Latin for "the unknown by the more unknown") describes an explanation that is less familiar than the concept it would explain.

An example would be: "The oven felt hot because of Fourier's Law." It is unlikely that a person unfamiliar with the hotness of ovens would be enlightened by a reference to a fundamental law of physics. Another example would be referencing Rayleigh scattering as an explanation for why the sky is blue, when a more apt explanation would be simply that air is blue.

That said, since these explanations could enlighten people in theory, ignotum per ignotius is not strictly a fallacy, but a criticism of an argument on rhetorical grounds, stating that such an argument is not useful in a particular context.

Ignotum per æque ignotum, meaning "the unknown by the equally unknown", is a related form of fallacy in which one attempts to prove something unknown by deducing it from something else that is also not known to be true.^[1]

• List of Latin phrases