The infinite regress is anticipated in one of your priors. You're playing a game. Variant A of an enemy attacks high most of the time, variant B of an enemy attacks low some of the time; the rest of the time they both do forward attacks. We have priors, which we can arbitrary set at any value. The enemy does a forward attack; here, we assign 100% probability to our observation of the forward attack. But let's say we see it out of the corner of our eye; in that case, we might assign 60% probability to the forward attack, but we still have 100% probability on the observation itself. Add an unreliable witness recounting the attack they saw out of the corner of their eye; we might assign 50% probability to that they're telling the truth, but 100% probability that we heard them. Add in a hearing problem; now we might assume 90% probability we heard them correctly, but 100% probability that we heard them at all. We can keep adding levels of uncertainty, true. Eventually we will arrive at the demon-that-is-deliberately-deceiving-us thing Descartes talks about, at which point we can't be certain of anything except our own existence. Infinite regress results in absolutely no certainty. But infinite regress isn't useful; lack of certainty isn't useful. We can't prove the existence of the universe, but we can see, quite obviously, the usefulness of assuming the universe does exist. Which is to say, probability doesn't exist in a vacuum; it serves a purpose. Or, to approach it another way: Godel. We can't be absolutely certain of our probabilities because at least one of our probabilities must be axiomatic.