Prototype Theory

Prototype theory is Eleanor Rosch's finding, from experiments in the 1970s, that everyday categories are graded resemblance to a central example rather than sets fixed by necessary and sufficient conditions. A robin is a better bird than an ostrich, and speakers agree closely on the ranking. Membership is a degree, which makes ordinary categories a poor fit for the crisp trees that Taxonomy assumes.

The evidence is behavioural. People verify typical members faster, list them first, acquire them earlier as children, and generalise new facts from them more readily — the results collectively called typicality effects. Rosch also identified a basic level, "chair" rather than "furniture" or "swivel chair", at which things are named by default. That default is what Plain Language means by the ordinary word.

Boundaries are fuzzy in a demonstrable way. William Labov asked people to name drawings of vessels and found the cup/bowl/vase line moved with width, with the presence of a handle, and with whether the vessel was said to hold coffee or mashed potatoes. There is no edge to locate, so no Ground Truth to appeal to.

This is why arguments about whether a hot dog is a sandwich never resolve. Each side reasons from a different prototype and neither is reporting a fact; the dispute is about category structure, settled by convention rather than discovery, which is why Naming a category is a consequential act. Ludwig Wittgenstein made the point earlier without experiments, describing game as held together by overlapping family resemblances rather than a shared essence.