Quotes from N. Chomsky and M. Halle  Sound Pattern of English (1968) p. 4.   (freely reformatted)

``General linguistics attempts to develop a theory of natural language as such, a system of hypotheses concerning the essential properties of any human language.  (They are) often referred to as linguistic universals.  The significant linguistic universals are those that must be assumed to be available to the child learning a language as an apriori innate endowment.

That there must be a rich system of apriori properties ... is fairly obvious. ... (since)

• Every normal child acquires an extremely intricate and abstract grammar, .. (which is) ... underdetermined by the available data.
• This takes place with great speed, under conditions that are far from ideal.
• There is little significant variation among children who may differ greatly in intelligence and experience.

• formal universals ... determine the structure of grammars and the form of rules ... and
• substantive universials ... the set of elements that may figure in grammars, .. (including) a particular fixed set of phonetic features.''