This article is an attempt to show that the study of presupposition mist distinguish two levels of analysis. At the first level, the presupposition is an element of the semantic content of the sentence. At the second, the presupposition links sentences or propositions, and then appears as inferable from another sentence or proposition. A first part is devoted to the presupposition as a sentence or a proposition and to the problems this conception leads to. It is intended to prove that the claim of a full separation between both levels is the only way to face these problems adequately, to the prejudice of the classical separation between presupposed and asserted contents. The second part brings forward two arguments supporting the claim of two separate levels, and moreover, shows the benefit of such a separation for the notion itself of presupposition.