Tautology

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Tautology illustrated by the web comic xkcd. "The first rule of Tautology Club is the first rule of Tautology Club."

A tautology is a logical statement which, by its very structure, must be true. For example:

A ∨ ¬A (A or not A)

In this example, A represents a statement, such as "God exists", "my shirt is white", or "all rabbits are green". But no matter what value we assign to A, the statement above is true:

Either God exists, or God does not exist.
Either my shirt is white, or my shirt is not white.
Either all rabbits are green, or else it is not the case that all rabbits are green.

Since tautologies are true a priori, many philosophers, including David Hume, consider they do not say anything useful about a posteriori synthetic propositions.

"Natural selection is a tautology"

Creationists often argue that natural selection is a tautology:

  1. "Natural selection" is defined as "survival of the fittest".
  2. Those individuals in a population who survive are defined to be the fittest.
  3. Therefore, "survival of the fittest" means "survival of those who survive" or "those who survive, survive".

"Those who survive, survive" is a tautology because there is no condition under which it could possibly be false.

It can be argued that "Natural selection" is neither defined as "survival of the fittest", nor is it a single statement; treating it as either could be a straw man.

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