 To bolster the claim that we do, in fact, understand complex expressions we never heard before, philosophers often appeal to unboundedness: although we are finite beings we have the capacity to understand each of an infinitely large set of complex expressions. Although there are dissenters—e.g., Ziff (1974)  and Pullum &amp; Scholz (2010)—the claim that natural languages  contain infinitely many complex expressions is  plausible.[17]  But it is equally plausible that nobody who reads this entry the first time has ever encountered this very sentence before, and consequently, the detour through cardinality considerations seems superfluous. Occasionally, the fact that natural languages are learnable is also used to argue for compositionality. This is not an independent argument: the reason it is remarkable that we can learn a natural language is that once we have learnt it our understanding is productive. If we could not understand expressions we never encountered before, without detailed empirical study we could not rule out the hypothesis that we learned the language in question by rote.