Argument from omniscience

Vizzini: I can't compete with you physically, and you're no match for my brains. Westley: You're that smart? Vizzini: Let me put it this way: have you ever heard of Plato? Aristotle? Socrates? Westley: Yes. Vizzini: Morons. An argument from omniscience (also allness, absolute thinking) occurs when somebody thinks they know literally everything or know everything about the subject at hand. The fallacy is commonly expressed with words like "all", "everyone", "no one", "everything", "always", or "never".

The problem with such an argument is twofold: first, that an arguer would need omniscience to know about everyone's beliefs or about everything in existence, and second, because the arguer does not have omniscience, they are often wrong and counterexamples to the generalization exist.

The argument is an informal fallacy and is theoretically the opposite of an argument from ignorance; however, because it relies on things that the speaker cannot know, it functions as an argument from ignorance and fails for the same reasons.

This fallacious form of reasoning often comes in two fantastically fallacious flavours — Alleged Certainty and Amazing Familiarity. In the case of Alleged Certainty, a claim is primed with language, such as "every", "everyone knows", and "we all know", that suggests that something is certainly the case, without providing a solid reason or evidence. This allows them to exploit the supposed obviousness of their point, as the audience, not wanting to feel like they've ignored the obvious, are convinced to agree. It is similar to the Appeal to popularity.

Amazing Familiarity occurs when a claim is made about someone or something, but the information necessary to support this claim is impossible to obtain or would require omniscience on the part of the speaker. This overlaps with wishful thinking, as the claim will often involve projections on the part of the speaker.

Examples
An important example is that of overprecision.


 * Everyone knows that.


 * All people believe in something.


 * All students love studying.


 * The president is a good man and would have never cheated on his wife, and has never cheated in anything in the past.

In the first three examples, there may be people who don't — and unless the arguer can prove that no such people exist, then their argument is flawed. In the fourth, the speaker would have to be extremely close to the President — and watched him for almost his whole life — to be able to defend this point.