The primary test field for theories of how the world can be known by sight has, over the centuries, been overwhelmingly the general problem of space perception, in particular, distance perception. The first step in addressing this problem is a question of the most fundamental kind: What is space? It is not difficult to appreciate that resolution of the scientific problem of how space is perceived depends ultimately on the correctness of the scientific presumption of what space is apropos perception and action. The question can be refined: What kind of concept is “space”? The most dominant answer is that space is a mathematical concept. A subordinate, but nonetheless influential answer is that space is a physiological/psychological concept. A third answer, hardly ever entertained, is that space is a biological/ecological concept.