Fallacy?

Equivocation

One word carries two meanings, and the argument slides from one to the other.

Also called: Shifting meaning · Bait and switch

The test

Does the key word mean the same thing in the evidence and in the conclusion?

Words such as evolution, information, new function and transitional each cover a modest meaning and a sweeping one. The modest meaning is often observed; the sweeping one is the claim in question. An argument equivocates when evidence for the first is counted, without comment, as evidence for the second.

This is the pattern most often found on this site, and it is the reason many claim cards carry two grades. Splitting a claim into its modest and sweeping senses is simply the cure for equivocation, applied before grading.

Seen on both sides

In mainstream arguments
It would be equivocation to cite observed evolution (a change in gene frequencies in a population) as support for evolution in the sense of all living things descending from one ancestor.
In critical arguments
It would be equivocation to reply that evolution is “just a theory,” using the everyday sense of theory (a guess) against the scientific sense (a tested explanatory framework).
Not a fallacy when

The meanings are kept apart, and the evidence is shown to support the stronger one on its own terms.

Diagnosed on this site

  • Pillar 4 · Mutation & New InformationClear case
    In the pillar as a whole

    The pillar's central word, information, is used for both a change in an existing gene and the origin of a new one. Most of the disagreement on these cards turns on keeping the two apart.

  • Claim № 4.01 · Mutation & new informationClear case
    In the claim as taught

    “New genetic information” covers two things: a new use for an existing gene, which has been observed (citrate), and new genes and folds, which have not. Evidence for the first is counted as evidence for the second.

  • Claim № 4.01 · Mutation & new informationPossible
    In evidence: Lenski's E. coli: citrate use

    Often reported as “a new function evolved.” What was observed was an existing transporter expressed under new conditions. Both descriptions are accurate; only the second says what kind of novelty it was.

  • Claim № 4.02 · Duplication & divergencePossible
    In the claim as taught

    “A new function” can mean a weak side-activity made strong, or a function with no precursor at all. The evidence supports the first meaning.

  • Claim № 3.03 · Embryos & developmentPossible
    In the claim as taught

    “Embryos look alike” can mean the strong claim that development replays ancestry, which has been abandoned, or the modest hourglass claim, which is defended. Textbook treatments often let the vividness of the first carry the second.

  • Claim № 4.03 · How rare are working proteins?Clear case
    In evidence: Function in random sequences

    Binding a small molecule is counted as “function,” and then set beside estimates for folded enzymes. The two numbers measure different things.

Further reading

  1. Walton DN. Informal Logic: A Pragmatic Approach. Cambridge University Press, 2nd ed. (2008).