[…] we must require of such a linguistic theory that it provide for (14) (i) an enumeration of the class 's1, s2'...of possible sentences; (ii) an enumeration of the class 'SD1, SD2'...of possible structural descriptions; (iii) an enumeration of the class 'G1, G2'...of possible generative grammars; (iv) specification of a function 'f' such that 'SDf(i,j)' is the structural description assigned to sentence 'Si' by grammar 'Gj', for arbitrary 'i, j'; (v) specification of a function 'm' such that 'm(i)' is an integer associated with the grammar 'Gi' as its value (with, let us say, lower value indicated by higher number) [...] we shall call a theory of linguistic structure that meets conditions (i-v) an 'explanatory theory'.
- Chomsky (1969), Pag. 34