Varying probabilities for events to happen, and ranges of error in human reasoned judgements, make outcomes uncertain. In science, all critical conditions must be fulfilled for certainty, presupposing exact and demonstrable knowledge of causes.
Uncertainty and unpredictability have become significant factors in international economic relations. It is often uncertainty, together with insecurity, that engenders international conflict.
The 'uncertainty principle' or 'principle of indeterminacy' discovered by W Heisenberg states that the position and velocity of an object cannot be measured accurately at the same time. Only for the exceedingly small masses of atoms and subatomic particles does the product of the uncertainties become significant for research purposes. Nevertheless, for practical purposes the uncertain coordinate of velocity with position for the atom does not prevent its splitting to release nuclear energy. The uncertainty principle in quantum mechanics has nothing to do with knowledge or action on the human level. However insofar as this principle illustrates the meaninglessness in nature of a concept of theoretical exact simultaneous knowledge of object position and location, it raises epistemological questions as to the basis for action which also were discussed by Plato, Aristotle and other philosophers. Chief amongst these are the idealists who fall into solipsistic and other subjectivist errors of divorcing philosophy from life with the view that all knowledge is uncertain, therefore the only logical action is to think and refrain from action, or more logically, to refrain from acting and thinking. The problem can perhaps be restated. Knowledge is certain in terms of its eventuality. Complete knowledge is unachievable. The level of knowledge must be determined that is appropriate to specific actions. For example, relief operations need not be delayed until a count of casualties is made; and justice need not require all the evidence, only a sufficiency. There are also signs which may not be certain but are indicative: for example, when dark clouds gather rain is expected; or when a country militarizes aggression may be anticipated. Thus action may be based on uncertain but probable knowledge; reasonable certainty.