When evidence underdetermines theory choice, philosophy of science names the missing ingredient extra-evidential criteria, not more data

Underdetermination of theory by evidence is a settled term of art, not a vault coinage. The Stanford Encyclopedia of Philosophy defines the situation as one where "for any body of evidence confirming a theory, there might well be other theories that are also well confirmed by that very same body of evidence." [Tier 2] When rival theories fit the same data equally well, the evidence has run out before the choice is made — and something other than the evidence must settle it, or the choice stays open.

Philosophy of science has a standing catalogue of what that something is. Quine appealed to pragmatic virtues — "simplicity, familiarity, scope, and fecundity" plus conservatism — as the criteria that break ties evidence cannot. Mary Hesse argued that underdetermination is precisely why "non-logical" and "extra-empirical" considerations must enter theory choice. Lakatos and Feyerabend pressed the darker reading: absent any such arbitrating factor, the gap between successful and unsuccessful theories collapses into the advocates' "talent, creativity, resolve, and resources" rather than the evidence. [Tier 2, via SEP] Across all three the shape is the same — the missing ingredient is an extra-evidential criterion, a rule for choosing that the data itself does not supply.

This gives the vault's "arbitration" framing real standing rather than metaphor. Grujicic's "no arbitration between" competing RSA similarity measures (claim-representational-similarity-underdetermines-mechanism), the empirical metric-reversal in claim-similarity-measure-choice-reverses-neural-representational-conclusions, and the unvalidated "bridge = knowledge gap" reading in claim-bridge-detection-lacks-pkg-validation are all cases where a validated substrate leaves an interpretive choice open — the pattern named in observation-substrate-laundering-across-marr-levels. Whether one extra-evidential criterion unifies all of them is the open thread at question-arbitration-as-missing-ingredient-across-underdetermination; this note establishes only that the vocabulary is real and load-bearing.