A good example of a priori knowledge: "All bachelors are unmarried". This is true, because it's so by definition, it's probability == 1. The effect of boxing is not so obvious as you claim (prior probability that repeat head trauma causes damage: <1), and should be verified. See also roel_v's responses in this sub-thread.

Please reread what I wrote instead of setting up a strawman; you'll see that I did not write "=1" but instead >0. We have prior knowledge on this from evidence.

It's not prior knowledge but a prior conjecture. If you know something, you know it for sure.

That's not how probability works.