Even the most elementary considerations in logic may lead to conundrums. Suppose that we wish to define the notion of “line.” We might say that it is the shortest path between two points. This is not completely satisfactory because we have not yet defined “path” or “point.” And when we say “the shortest path,” do we mean that there is just one unique shortest path? And why does it exist? Every new definition is, perforce, formulated in terms of other ideas. And those ideas in terms of other ones. Where does the regression cease?