None of the four: code, tests, spec, people's memory, are the single source of truth.

It's easy to see them as four cache layers, but empirically it's almost never the case that the correct thing to do when they disagree is to blindly purge and recreate levels that are farther from the "truth" (even ignoring the cost of doing that).

Instead, it's always an ad-hoc reasoning exercise in looking at all four of them, deciding what the correct answer is, and updating some or all of them.