There are too many contingencies in life. In the real world, an agent can never be 100% sure.
"Plan A will get us to the airport in time, as long as my car doesn't break down or run out of gas, and I don't get into an accident, and there are no accidents on the bridge, and the plane doesn't leave early, and there's no earthquake, comet, UFO abduction, ...."
But, an agent can make a Rational Decision.
Rational Decision: Depends on both the relative importance of
various goals and the likelihood, and the degree to which, they will be achieved.