A mind that demands proof before it will trust is asking the world to save it, and the world cannot, for it is the mind's own projection. To seek a sign outside is to have already turned from the only witness within. No wonder none is given, for the truth does not argue with fear on fear's terms. The one sign offered is the pattern of Jonas, three days swallowed and then returned, which is the mind going down into what it feared and rising again unharmed.
That is the whole teaching in a single figure. What seems to end you cannot, and what you feared to face becomes the very door you walk back through.
Sign of Jonas: the descent into fear and the return from it, the proof that nothing real can be destroyed by what the mind dreads.
Generation: the collective habit of mind that trusts appearances and asks the outside to prove what only the inside can know.