A chain of vetoes is a redundancy, and like any redundancy it protects only as far as its parts fail independently. On submarine B-59 the two keys that agreed to fire were held by men in the same metal barrel: the same heat, the same carbon dioxide, the same missing radio. The third key held because Arkhipov sat in a different seat in the chain (flotilla commander, not crew), which Ord calls "the purest luck". Turning that luck into design means choosing keyholders for decorrelated failure: different information, different incentives, different conditions. A second key held by a copy of the first adds ceremony, not safety. This is the test P8 applies to a second world (C-71XFMPBMAP), applied to a decision, and it is why diversity of intelligence is a safety property and not a courtesy.

Evidence 2 passages

groundsThe Precipice, Toby Ord
It was out of radio contact, so the crew did not know whether war had already broken out.
groundsThe Precipice, Toby Ord
Firing the nuclear weapon required the agreement of the submarine's political officer, who held the other half of the firing key. Despite the lack of authorization by Moscow, the political officer gave his consent. On any of the other three submarines, this would have sufficed to launch their nuclear weapon. But by the purest luck, submarine B-59 carried the commander of the entire flotilla, Captain Vasily Arkhipov, and so required his additional consent. Arkhipov refused to grant it.

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