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Similarity is a relation, more or less formal, depending on context. We naturally associate similar things, as in rhyming; and especially as having similar uses, for need detects likeness: necessity is the mother of resemblance. Euclidean geometry, which is much concerned with shape, does away with “similarity” in the general sense, narrowing its meaning to mere “sameness of shape.” In certain contexts, as with words and living things, it is “relatedness” that can be reduced to a fairly precise meaning: “relation by descent,” which can (as a rule, in principle) be exactly diagrammed. Without losing the useful breadth of “similarity” in these contexts—admitting, therefore, such categories as appearance, sound, shape, function, behavior—one can try to limit it to refer to properties which can be measured. In the case of living things, the modes of classification by (familial) relatedness and (measurable) similarity have respectively been given the unlovely names of cladistics (referring to branching of a genealogical tree) and phenetics (referring to appearance).

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