Uncertainty

Uncertainty is an inevitable part of the assertion of knowledge, see Bayesian probability

Examples where uncertainty is important:

Uncertainty is designed into games, most notable, gambling where chance is central to play.
In physics in certain situations, uncertainty has been elevated into a prinicple, the uncertainty principle.
In weather forcasting it is now commonplace to include data on the degree of uncertainty in a weather forecast[?].
Uncertainty is often an important factor in economics.

Further Reading