Fallacy?

Affirming the consequent

If H, then we'd see E. We see E. So H. This only works if no rival explanation also predicts E.

Also called: Fit taken as proof

The test

Would a rival explanation also predict this evidence? Was the rival actually tested?

In deductive logic this is a formal fallacy: the conclusion does not follow. In science the same shape is everywhere, because hypotheses are tested by their consequences. What rescues it is comparison. Evidence supports a hypothesis to the extent that the hypothesis predicts it better than the alternatives do. Elliott Sober's work on the logic of evolutionary evidence makes this point carefully, and it cuts both ways.

So this label rarely means “wrong.” It means “not yet a test.” On this site it appears on both readings of the evidence: wherever an argument shows that the data fit, and stops there.

Seen on both sides

In mainstream arguments
It would affirm the consequent to argue: common descent predicts shared structures; shared structures exist; therefore common descent, when a common designer reusing good solutions would predict shared structures too.
In critical arguments
It would affirm the consequent to argue: design predicts functional complexity; organisms are functionally complex; therefore design, without showing a prediction that undirected processes do not also make.
Not a fallacy when

The evidence was predicted by the hypothesis and not by its serious rivals. Comparing how well rivals predict the data is how inference to the best explanation works.

Diagnosed on this site

  • Pillar 2 · The Fossil RecordPossible
    In the pillar as a whole

    On both readings, the fossil pattern is shown to fit the favoured explanation more often than it is used to test it. The cards ask, in each case, what result would tell the explanations apart.

  • Pillar 3 · Common DescentPossible
    In the pillar as a whole

    Both common descent and common design predict nested similarity. The evidence that would favour one over the other is the evidence each card asks for.

  • Claim № 3.01 · HomologyPossible
    In the alternative reading

    “Exactly what we would expect from a single designer” shows that the design reading fits the pattern. It still needs a prediction that common descent does not make; the card's final requirement asks for one.

  • Claim № 2.01 · Transitional seriesClear case
    In the claim as taught

    Descent predicts intermediate forms; intermediate forms exist; so descent. A view in which a designer combined features across kinds would also expect mosaics. The test that would separate the two, ancestry itself, is the one Gee argues fossils cannot supply.

  • Claim № 2.01 · Transitional seriesPossible
    In the alternative reading

    “Mosaic creatures are what we would expect from a designer” has the same form. The reading fits the data, but fitting is not yet a test. It needs a prediction that descent does not make.

  • Claim № 3.02 · The tree of lifePossible
    In the alternative reading

    A patchwork of similarities is said to be what a designer reusing components would produce. The card itself calls this a hypothesis with its own predictions. Until they are tested, it is a fit, not a test.

  • Claim № 3.02 · The tree of lifePossible
    In source: Theobald DL. “A formal test of the theory of universal common ancestry.” Nature 465:219–22…

    Koonin and Wolf argued that the test also favours common ancestry for sequences made similar by convergence. If so, it cannot rule out the main rival it was meant to exclude. Theobald disputes this.

  • Claim № 2.02 · Cambrian explosionPossible
    In the alternative reading

    Abrupt appearance is said to fit creation “at least as naturally.” Fit is shown, but not a test. A creation hypothesis needs a prediction here that gradual descent does not make.

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

    Similarity in mid-development is attributed to descent. A shared engineering constraint on building a vertebrate body would predict it too, as the alternative reading notes.

  • Claim № 3.03 · Embryos & developmentPossible
    In the alternative reading

    Different early routes converging on a common body plan are said to look like design. That is a fit. It needs a prediction about development that common descent does not make.

Further reading

  1. Sober E. Evidence and Evolution: The Logic Behind the Science. Cambridge University Press (2008).
  2. Lipton P. Inference to the Best Explanation. Routledge, 2nd ed. (2004).