A pleonasm (word redundancy) plus repetition that amounts to a circular definition. It can be defined in a number of related ways:
- the repetition of an idea in a different word, phrase, or sentence (e.g., “The annual psychology conference is held once a year”; “A tautological statement is a statement that is tautological”),
- a statement whose predicate repeats its subject in whole or in part,
- a statement of anything as the cause, condition, or consequence of itself, and
- a statement in which the conclusion is equivalent to the premise.
Perhaps the most cited example of a tautology in science is the phrase “the survival of the fittest” formulated by Herbert Spencer (1820-1903) some years before the publication of the Origin of the species (1859), and subsequently more or less accepted by Darwin as a catch phrase for his theory of natural selection: ‘fittest’ means ‘fittest to survive’ and hence the expression is just a roundabout way of saying what survives survives. Since the expression is clearly circular and therefore vacuously true, natural selection cannot be falsified. Those who contend that it is not a tautology, point out that fitness is more than just survival. For Darwin, it meant not those who survive, but rather those who could be expected to survive, due to their adaptations and functional efficiency, when compared to others in the population. As such, this is not a tautology as the conclusion differs from the premise. If it is, then so is Newton’s second law of motion (Force = mass x acceleration or F=ma). The problem is that adaptation and natural selection have been linked together unquestioningly in the Modern synthesis. The solution is to provide independent definitions of the two terms and then check to see if natural selection always leads to adaptation. Are there other factors than natural selection and what are the consequences of natural selection than just adaptations to specific environments (e.g., warm bloodiness that cuts across ecological boundaries)?
See Adaptation, Modern synthesis, Newton laws of motion, Operationalism, Theory of natural selection