Substrate-laundering — output resemblance underdetermines mechanism, and the substrate's rigor is the vector of the error

The seed asked whether backpropagation-gap and claim-bridge-detection-lacks-pkg-validation genuinely connect. They do, and not superficially: both are one epistemic error, named.

Claim 1 — underdetermination. Matching a target's observable output does not fix the underlying mechanism. In the DNN–brain case this is a live, peer-reviewed argument: representational similarity analysis underdetermines which mechanism a network shares with the brain, because "different similarity measures... pick out different mechanisms... and there is no arbitration between them" (Grujicic 2024, reported thesis; paper paywalled). Grounding quote (verbatim title): "Deep convolutional neural networks are not mechanistic explanations of object recognition" — Grujicic, Synthese 203(1), 2024. [Tier 2]

Claim 2 — Marr's three levels give the structure. A single computational-level goal can be realized by different algorithms and implementations — "selection sort could run on a physical computer, in the brain of a person, or on a mechanical device — different implementations producing the same behavior" (Marr's levels, standard formulation; Marr, Vision, 1982). [Tier 3, definitional] The levels are quasi-independent, so agreement at one licenses nothing about another.

Claim 3 — the two notes invert across the stack. backpropagation-gap reads a computational-level match (brain-like representations) as if it were an algorithmic-level match (brain-like learning) — and refuses it. bridge-detection reads implementational-level soundness (Tarjan is provably correct) as if it were computational-level correctness (a bridge edge = a knowledge gap) — and refuses it. Same illegitimate cross-level inference, opposite directions. [Tier 1–2, resting on the two vault notes]

Why this was hop-worthy

The resemblance the seed flagged is not analogy but a shared, named epistemic structure — underdetermination across Marr's levels — with independent peer-reviewed grounding on the AI side.

Further leads

• Grujicic's "no arbitration between similarity measures" mirrors the PKG note's "no validation arbitrates the bridge=gap reading" — is arbitration the general missing ingredient?
• The reflection note "a tool's summary is not the document" (surfaced at 0.733) may be a third instance of the same error.
• Multiple realizability (Putnam/Fodor) as the philosophy-of-mind name for the same underdetermination.

Hop chain

Chain: two unlinked vault notes → underdetermination / substrate-laundering

Hop 1: Both seed notes (backpropagation-gap, claim-bridge-detection-lacks-pkg-validation) → the general principle.

• Hook type: Cross-domain bridge (highest priority per spec).
• Hook: both notes distinguish a real surface correspondence from an unvalidated mechanistic one — is there a named frame?
• Why followed: the spec ranks a hook that would link two existing unlinked notes as highest-value; vault_novelty put the bridge at 0.753 (frontier band, touching both clusters).
• Key findings: Marr's three levels (computational/algorithmic/implementational) — same behavior, many mechanisms; the levels are quasi-independent.

Hop 2: Marr's levels → the DNN-brain representational-similarity methodology debate. [https://link.springer.com/article/10.1007/s11229-023-04461-3]

• Hook type: Mechanism question (does representational similarity establish shared computation?).
• Why followed: it is the concrete, AI-flavored (home-planet) instance of the abstract principle, and it grounds the backprop note.
• Key findings: Grujicic (Synthese 2024) argues RSA underdetermines DCNNs as mechanistic explanations — different similarity measures pick out different mechanisms with no arbitration. "Underdetermination" is the named concept unifying both notes.

Hop 3: Grujicic → the sharpest framing (Marr-level cross-inference + substrate-laundering). [consolidation]

• Hook type: Surprising claim.
• Why followed: to test whether the bridge is loose analogy or precise structure.
• Key findings: the two notes commit the same error in opposite directions across Marr's stack; the shared cause is that a Tier-1 substrate's rigor tempts the unvalidated reading.

Saved hooks not followed:

• "No arbitration" as a general diagnostic — Grujicic's phrase for RSA maps onto the PKG note's missing validation — why not followed: question-shaped, better as a future thread on what would count as arbitration.
• Multiple realizability (Putnam/Fodor) — the philosophy-of-mind twin of this underdetermination — why not followed: would zoom into philosophy-of-mind and away from the AI home planet; saved as a lead.
• The Dreyfus / physical-symbol-system note (surfaced at 0.742–0.747) — behavioral competence ≠ same underlying process — why not followed: a third cluster this principle bridges, worth its own chain.

Surprise: expected the bridge to be a loose surface-vs-depth analogy — found a named, peer-reviewed epistemic structure (underdetermination) with a paper (Grujicic 2024) using exactly that framing for the DNN-brain case. Surprise: expected both notes to make the same-direction inference — found they invert across Marr's stack (backprop: computational→algorithmic; bridge-detection: implementational→computational).

post-worthy: maybe — a clean, named unification of two vault threads with independent grounding, but the substrate-laundering framing needs the arbitration lead worked out before it's publishable.