Updated: Mar 17
We use ideas about inwardness, depth, and holes to describe knowledge: “in the know,” (in—know), “deep understanding” (down—knowledge), “depth of knowledge” (down—knowledge), “depths of your soul” (down—soul—knowledge), “made a strong impression” (in—knowledge), “a case in point” (in—round—reality), “fill me in” (in—knowledge), “let it sink in” (in—knowledge), “what it comes down to” (down—reality), “double down” (down—down—believe), “tripling down” (down—down—down—knowledge), “food for thought” (in—knowledge), “deep dive” (down—down—knowledge), “holy cow” (in—cow—realization), “holy crap” (in—solid—realization), “holy moly” (in—realization), “in a nutshell” (in—solid/container), “down pat” (down—know exactly), “you don't have to keep rubbing it in” (in—in—in—I already know) and “I can’t fathom” (not down—lack of knowledge).
At least 20 popular English expressions suggest we relate knowledge to downwardness and inwardness. Other languages are probably similar. If so, the phenomenon should be thought of as a result of the way the brain and mind function rather, as a side effect of its structure, rather than ever having a direct adaptive purpose in itself, due to the fact that the amount of agreement between people it would take to accomplish such a thing is too widespread and persistent for it to be explained any other way. This assumes phrases relating knowledge to upwardness and outwardness cannot be shown to be as common as those listed above. "Are you up on" (up—knowledge) is an example.