It occurred to me one could draw a basic relation between certainty and learning that I drew between money and wealth. If one is perfectly certain, one is incapable of learning, which I will define as perceptual movement. If one doubts everything–really, the acceptability of treating anything as a “fact”–then nothing is accumulated; there is no residual one might term “knowledge”.
The task, then, is to balance the two. It is a bit like walking. You have believe that in voluntarily falling forward the other leg will catch you. And one can treat all those bits we call facts as useful until they prove otherwise. Practical knowledge is what is useful, which in turn normally means observable and repeatable.