This is one of the key insights of Bayes' rule: Given what you knew, and what you saw, the maximally accurate state of belief for you to be in is completely pinned down, and while it's quite difficult to find in principle, we know how to find it in practice\. Humanity has uncovered some of the laws of reasoning, and they are captured by Bayes' rule\. If you want your beliefs to become more as you observe the world, Bayes' rule gives some hints about what you need to do\.
It's not totally clear what the antecedent of this "it's" is. (Because "it's" often means "it is the case that")