It has become customary for linguists (especially generative linguists) to draw a distinction (initially rather unnatural for philosophers of science) between “internal” and “external” evidence. Usually classified as internal are data on the cooccurrence and alternation of linguistic elements in some language, as well as such systemic considerations as formal simplicity, economy, and the like. External evidence is everything else: the use of phonemes in rhyme schemes, patterns of acquisition, comparison to other languages, speech errors, dialect differences, historical change, and so on. The distinction is usually made invidiously--only internal evidence is probative--or defensively--external evidence, or at least some types of external evidence, are relevant and useful.

The distinction arises at two quite different points in the investigation of language. It arises first in the division of labor between linguistics and other fields.