> We accomplish this by forming concepts such as "ledge", "step", "person", "gravity", etc., as we experience them until they exist in our mind as purely rational concepts we can use to reason about new experiences.
So we receive inputs from the environment and cluster them into observations about concepts, and form a collection of truth statements about them. Some of them may be wrong, or apply conditionally. These are probabilistic beliefs learned a posteriori from our experiences. Then we can do some a priori thinking about them with our eyes and ears closed with minimal further input from the environment. We may generate some new truth statements that we have not thought about before (e. g. "stepping over the ledge might not cause us to fall because gravity might stop at the ledge") and assign subjective probabilities to them.
This makes the a priori seem to always depend on previous a posterioris, and simply mark the cutoff from when you stop taking environmental input into account for your reasoning within a "thinking session". Actually, you might even change your mind mid-reasoning process based on the outcome of a thought experiment you perform which you use to update your internal facts collection. This would give the a priori reasing you're currently doing an even stronger a posteriori character. To me, these observations above basically dissolve the concept of a priori thinking.
And this makes it seem like we are very much working from probabilistic models, all the time. To answer how we can know anything: If a statement's subjective probability becomes high enough, we qualify it as a fact (and may be wrong about it sometimes). But this allows us to justify other statements (validly, in ~ 1-sometimes of cases). Hopefully our world model map converges towards a useful part of the territory!