I think Jon's post was definitely getting us somewhere. I had a bunch of knee jerk reactions I wanted to let calm down (including the one Nick brought up).
I think at this time the most important thing I want to emphasize is that the distinction in question is one found via experimental investigation, not arm chair speculation. It isn't an arbitrary distinction Nick and I "find it meaningful" to make, it is a distinction revealed empirically.
Water exposed to electrolysis makes gas. If you arrange your apparatus correctly, you can collect gas separately off each electrode. If you investigate those gasses you find that they behave differently in numerous ways. At that point, it is weird to say that chemists "find it meaningful to distinguish" the two gases. It isn't that saying that is, strictly speaking, incorrect, but it implies an arbitrariness about the whole thing.
But this is actually a bigger distinction than that, logically speaking.
When ethologists started asking "why is that animal behaving in that fashion?" they used a variety of different methods, and found that some methods produced different types of answers than other methods. Sometimes when that happens, you keep trying to do science and end up with a jumbled mess, but that's not what happened here. Time and time again set-of-methods A converged one answer, while set-of-methods B converged on a different one. And in decades of investigation by a field of Biology recognized well enough to get three people Nobel Prizes, never once did the two sets of methods settle upon the same answer. At that point, the reasonable conclusion is that set-of-methods A is measuring one thing, while set-of-methods B is measuring a different thing.
Looking at the methods and the findings across numerous, numerous studies: Set-of-methods A seems to point at the evolutionary function of the behavior in question, while set-of-methods B seems to point at the immediate goal of the organism. We could imagine living in a world where those were not different things; many early evolutionary theorists thought no such distinction would be found; and even some current evolutionary theorists talk as if no such distinction exists (exasperating those of us steeped in the relevant literature). But, it turns out, the distinction is there.
So this is less like the distinction between hydrogen and oxygen, and more like the distinction between PH and surface tension. They are distinguished by fundamentally different methods of investigation. You could imagine a "possible world" in which PH and surface tension perfectly coincided, but that isn't the world we live in. Yes, chemistis "find it meaningful to distinguish" between PH and surface tension, but phrasing it that way suggests the issue is being approached oddly.
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Eric P. Charles, Ph.D.
Department of Justice - Personnel Psychologist
American University - Adjunct Instructor