Probabilistic models are what I was referring to; all probabilistic models should produce a prediction, and also a measure of confidence. That measure of confidence can never be 100%. Similarly, you can never have an incontrovertible belief that is not a mathematically/logically-constructed fact 'outside' of the messy real world in some abstract chamber walled off from anything that is like a real life observation, no matter how certain. Basically, you can't have P(A) = 0 or 1 as your prior probability, because then you break Bayes' rule in the sense that new evidence is meaningless - https://en.wikipedia.org/wiki/Cromwell%27s_rule