Proximate and Ultimate Causes

In 1961, Ernst Mayr published Cause and Effect in Biology in Science, restructuring the conceptual foundations of an entire discipline with a distinction so simple its profundity was easy to miss. Biology, Mayr argued, requires two kinds of explanation, not one. The proximate question asks how a trait works — the aerodynamics of a wing, the chemistry of pigmentation, the neural circuits that coordinate flight. The ultimate question asks why the trait exists — the evolutionary history of selection pressures that, across millions of years, produced this specific solution to this specific problem. Both questions are legitimate. Both require different methods, different evidence, different standards of satisfaction. Confusing them, Mayr argued, had produced a century of errors in biology — category mistakes that sent entire research programs down unproductive paths.