In classical philosophy, there were various ways in which things could be distinguished. The merely logical or virtual distinction, such as the difference between concavity and convexity, involves the mental apprehension of two definitions, but which cannot be realized outside the mind, as any concave line would be a convex line considered from another perspective. A real distinction involves a level of ontological separation, as when squirrels are distinguished from llamas (for no squirrel is a llama, and no llama is a squirrel). A real distinction is thus different than a merely conceptual one, in that in a real distinction, one of the terms can be realized in reality without the other being realized.