Fallacies: F (Part 1)

Core idea

The letter F opens with folklore and pseudoscience side by side: fairy rings, flying fish, the five-second rule, and the elaborate number-mysticism of biorhythms. The pairing is deliberate. Fairy belief and biorhythm belief are the same error in different costumes — a comforting pattern asserted without evidence. One uses spirits, the other uses arithmetic, and neither is true.

Why it matters

Pseudoscience is more dangerous than plain folklore because it borrows the look of rigour — cycles, formulas, slide rules, citations. Wilhelm Fliess built a whole system on the numbers 23 and 28 and even convinced Freud for a while; the maths was empty, because any two numbers without a common divisor can be combined to reach any total. When nonsense dresses as science, you need to check the substance, not the costume.

Key takeaways

Mental model

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Practical application

Check the formula, not the confidence

Biorhythm theory looks scientific: numbers, cycles, a slide rule. But Fliess's basic equation can express any integer, so it can be made to "fit" anything — which means it predicts nothing. When a system explains every outcome, it has no content. Ask what result it would rule out.

Watch for the believer who cannot lose

Conan Doyle accepted faked fairy photographs and rejected every patient explanation of how they were faked. A claim that survives all disproof is not strong; it is unfalsifiable. The believer's certainty is a feature of the belief, not evidence for it.

Test the convenient rule honestly

The five-second rule is folk wisdom shaped by appetite. Tested properly — sprinkling a floor with bacteria — food picks up contamination on contact, with no grace period. When a rule lets you do what you wanted anyway, run the real test before trusting it.

Example

A colleague swears by a biorhythm app: on "low" days they avoid big decisions, on "high" days they push hard. It feels validated, because good days and bad days do happen. But that is the trick — the app cannot fail. A good day on a "high" confirms it; a good day on a "low" is "an exception"; a bad day either way is explained too. A theory that absorbs every outcome is not reading your body — it is reading your willingness to believe. The honest test is to log predictions in advance and count how often they beat a coin toss.

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