A proof that asks for no trust. A machine checks every step, and either everything holds or nothing passes.
The difference from ordinary review is the difference between a taster and a scale. The taster is experienced but human: gets tired, misses things, sometimes wants the result to be true. The scale wants nothing. It shows what is.
That is exactly why formal verification matters in the model era. Text from a model can sound flawless and be wrong in the middle. Once it passes Lean, the question of who wrote it stops existing. A verified step is a verified step.
Where you meet it
In mathematics, when a result from an AI system seeks legitimacy. In critical software: avionics, medical devices, operating system kernels. And more and more in the AI safety conversation - as the only kind of answer that does not depend on the honesty of whoever is answering.