Definition
Contingency is the principle that history could have unfolded very differently — that the actual sequence of events is one of many plausible paths, not the inevitable consequence of deeper laws. A contingent outcome is one where the result depends on the specifics: who happened to be king, which fleet caught the favourable wind, which microbe reached which port first.
Harari uses contingency to demolish the comforting Whig story that progress flows in a single direction. There is no historical "should." The fact that an outcome happened is not, on its own, evidence that it had to. Replay the tape, and a different religion, a different empire, and a different idea of money might have come out on top.
Why it matters
How it works
Contingency operates at junctures. At each juncture, the prior path has narrowed the available options, but several still remain. Which option wins is decided by accidents — a battle, a plague, an individual's idiosyncratic choice — whose outcomes may amplify into civilisation-scale differences. After the juncture, the winning path looks obvious in retrospect, because all the alternatives have been pruned.
The trick the historian must perform is to recover those pruned branches. Harari does this by asking, at the Cognitive Revolution, the Agricultural Revolution, and the rise of the universal orders, what plausibly might have happened instead — and what the dominant story takes for granted that, on inspection, was not foreordained at all.