Up or down

The metaphor of depth in thinking arises from the assumption that there are levels of understanding. The idea works like this: the deeper the thought, the more surface subjects it cuts across. Depth, in this sense, implies a broad range of thinking. We also use metaphors of height to confer the same idea, e.g. high-level thinking, bird's eye view, ten-thousand foot level of detail, forest for the trees. To me this indicates that some people picture hierarchies as narrowing toward the top, and some people picture hierarchies as narrowing toward the bottom.