Tautology

The First Rule of Tautology Club is the first rule of Tautology Club. A tautology is a tautological statement that is tautologous in nature. Why? Because we (tautologists) told you that because we told so.

But seriously
A tautology is a statement that, by virtue of logic and nothing more, is true, but in such a way that it contains no useful information: "The red bicycle is red." Sometimes this can extend beyond simple logic to obvious definitional facts: "The red bicycle has two wheels."

Sometimes tautologies are hard to extricate and identify when they are buried in masses of argumentative prose. In other situations, it may be obscured by experimental design.

One particular kind of tautology is "assuming the antecedent", also known as "circular logic". This is often engaged in by Bible thumpers, who will justify their argument with Biblical citations and take the Bible's own assertions as proof of its being true. A classic example of this, which one will ultimately run into in any lengthy discussion of the Bible with a fundamentalist Christian, is "I know what the Bible says is true, because the Bible says it is."

Formal definition
"given A (and whatever puffery is there to hide the fallacy), then (eventually) A".

Reduced to the statement "A implies A", this is actually the simplest form of tautology.

Legitimate uses
Tautologies can be useful for emphasizing important facts, or putting them in more useful forms. Tautologies can also be used as a starting point for some logical methods (in particular, most formal definitions of mathematics depend in part on the tautology "A=A" to help define the concept of mathematical equality). And naturally, any assumption that disagrees with a tautology is false.

A paradigm example: “Que será será”
Doris Day’s innoxious “tautological fatalism”: