Re: Definition required
"Well sure, unknowns can kill you, but until it has presented itself it is a mere possibility, not an actual threat. You can't prepare, or even begin to prepare, for possibilities, for unknowns, you simply don't know how. By attempting the impossible you are reducing the effect of your defensive resources."
Two words: contingency planning. Learn to expect the unexpected.
"The other type of existential threat is the kind on which you have no input or control. In your Cold War example there was precisely zero you could do about that. Nothing."
That's assuming helplessness, but we can't think that way. Because, while the threat exists, it's hard to tell whether or not we CAN or CANNOT influence the threat. Indeed, in the Cold War, many times the actions of each side caused reactions on the OTHER side, giving concrete evidence of having an influence.