Hinton's "biologically implausible" is four specific objections — weight transport, a separate backward pass, discrete synchronized cycles, and required differentiability

The famous phrase compresses four distinct, checkable claims about cortex:

1. Weight transport — cortical feedback connections do not mirror feedforward ones, violating backprop's symmetric-weight requirement (Crick 1989 named a related but distinct objection — antidromic signalling, NOT weight transport; see claim-crick-1989-antidromic-not-weight-transport, resolved 2026-07-06).
2. No separate backward pass — backprop needs a distinct error- propagation phase and stored activities; neither is observed in cortex.
3. Timing — brains learn in real time from continuous streams; backprop wants synchronized discrete cycles.
4. Differentiability — every forward operation must be differentiable; biological computation need not be.

Each objection is an empirical claim about neuroscience, not a verdict on the algorithm — and the vault already holds the empirical counterpart: backpropagation-gap's Raugel et al. finding that brains and networks share representations but not learning mechanics. The objections are also the motivation for the alternatives now in the inbox (feedback alignment, forward-forward — captures pending). See moc-backpropagation-origins, claim-rhw-1986-demonstration-not-invention.