Though the exact definition is debatable, natural language is often contrasted with artificial or constructed languages such as Esperanto. The term “natural language” itself is not unproblematic, as all human languages are social conventions, not biological phenomena—what is biologically determined is only the child’s general capacity to learn one or several of these social languages.
The understanding of natural languages reveals much about not only how language works (in terms of syntax, semantics, phonetics, phonology, etc), but also about how the human mind and the human brain function. In linguistic terms, ‘natural language’ only applies to a language that has evolved naturally, and the study of natural language primarily involves native (first language) speakers.
Lyons distinguishes four different senses of the term “natural” in linguistics, but does not find any definition of it that would be sufficient to distinguish Esperanto from English. Yet for many linguists the term “natural” has a relatively well-established and well-defined meaning: a natural language is one that has native speakers. In sociolinguistics, native speakers constitute the basis of the speech community of every language; in psycholinguistics and Chomsky-inspired general linguistics, the proof of the naturalness of a language is that it is acquired as a first language in early childhood.
The theory of universal grammar proposes that all natural languages have certain underlying rules which constrain the structure of the specific grammar for any given language. Most constructed languages do not obey these constraints, and thus can be clearly distinguished.
While grammarians, writers of dictionaries, and language policy-makers all have a certain influence on the evolution of language, their ability to influence what people think they ‘ought’ to say is distinct from what people actually say. Natural language applies to the latter, and is thus a ‘descriptive’ rather than a ‘prescriptive’ term. Thus non-standard language varieties (such as African American Vernacular English) are as natural as standard language varieties (such as Standard American English).
In principle we can distinguish between a possible natural language that could be acquired as a first language, and an actual natural language that is actually spoken by native speakers. Although an infinite set of possible natural languages must exist, it is often assumed that we do not yet know all of the defining properties of this set. The main task of linguistics is to discover them. This would mean that L. L. Zamenhof, the creator of Esperanto, may have created a language, but it was not a readymade, natural language: if children now acquire Esperanto as a first language, they must, according to this line of thought, change it in some way or another, as happens in the nativisation and creolisation of pidgins and new sign languages.