Spiritually speaking: Difference between a chair and a stool?

I see the ogy with evolution, of course. But a low-backed chair still requires you sit on it in a specific position (unless you want to hurt your legs), while a stool can be placed in any position relative to the sitter's seat. Therefore, in terms of definition, there is a specific point. No such specific point exists between one individual and the next in an evolving species.