 The idea is that two individuals can know (or have, or use) different I-languages that generate exactly the same strings of words, and even give them exactly the same structures. This situation forms the basis of Quine’s (1972) infamous critique of the psychological reality of generative grammars (see Johnson 2015 for a solution in terms of invariance of ‘behaviorally equivalent grammar formalisms, to use Quine’s terminology, see also Nefdt 2021 for a similar resolution in terms of structural realism in the philosophy of science).