Count the assumptions each explanation requires
When two explanations fit the facts, count how many unverified assumptions each one rests on.
Key takeaways
- What it is: When two explanations fit the facts, count how many unverified assumptions each one rests on.
- Why it works: Each assumption in an explanation is an independent place the explanation can fail. An explanation with five assumptions has five places where the chain can break; one with two assumptions has two. Counting assumptions makes probability differences concrete — independent errors multiply, so the gap between explanations widens quickly.
- Evidence: Plausible mechanism, limited direct outcome data.
- Avoid: Treating simplicity as a trump card regardless of evidence — dismissing a complex but well-supported explanation because it feels complicated.
Why it works
Each assumption in an explanation is an independent place the explanation can fail. An explanation with five assumptions has five places where the chain can break; one with two assumptions has two. Counting assumptions makes probability differences concrete — independent errors multiply, so the gap between explanations widens quickly.
How to do it
- 1Write out the competing explanations as explicit chains: "this is true IF A, B, and C."
- 2Count the unverified assumptions in each chain.
- 3Ask which assumptions are actually checkable with current evidence.
- 4Favor the explanation with fewer unverified steps while remaining open to update.
What the evidence says
MechanisticThe parsimony preference is foundational in philosophy of science (Sober, Quine) and is encoded in Bayesian probability: simpler models receive higher prior probability because they make fewer independent bets. The formalization is well established in statistical model selection (AIC, BIC penalize unnecessary parameters).
Honest caveat: Parsimony is a prior, not a guarantee. Nature is sometimes genuinely complex; simpler explanations can also be wrong. The razor guides starting points, not endings.
Common mistake
Treating simplicity as a trump card regardless of evidence — dismissing a complex but well-supported explanation because it feels complicated.
IX Coach helps you lay out competing explanations for a situation and make the assumption count visible before you commit to one.
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