Suppose that we want to demonstrate (i), rather than simply assuming it.

(i) The language faculty exists as an autonomous system, i.e., as a system whose properties, and hence its workings, are not affected at all by pragmatics and other considerations outside the language faculty proper.

We should perhaps be cautious as to what linguistic phenomena we want to deal with since it is most likely the case that not every linguistic phenomenon will lead us to a successful demonstation of (i).