This article deals with a dilemma about the mass/count nouns’ distinction : is it arbitrary or ontological motivated ? This article attacks the prejudice stating that the autonomy of grammar excludes ontological motivation. In the first part, we will put forward two arguments for the grammatical autonomy of the distinction between mass nouns and count nouns. In the second part, we will argue that there are some concrete mass nouns whose quantification matches an independent ontological ground, and that there are some other mass nouns whose quantification draws lexical distinctions without any ontological ground. The formers are ontological motivated, the latter are not. Two conclusions will follow. Firstly : the idea that the autonomy of grammar is the presupposition of both motivation and non motivation. Secondly : a key to understand the strengths and the weaknesses of the debate about the motivation of mass/count nouns.