Uncertainty, on the other hand, is risk that is hard to measure.

You might have some vague awareness of the demons lurking out there. You might even be acutely concerned about them. But you have no real idea how many of them there are or when they might strike. Your back-of-the-envelope estimate might be off by a factor of 100 or by a factor of 1,000; there is no good way to know. This is uncertainty.

Risk greases the wheels of a free-market economy; uncertainty grinds them to a halt.