This paper upholds the position that the French prepositions à and de are not “empty” and do not fulfill only a syntactic function. After showing that the criterion of presupposition, used in some previous work, is not sufficient to differentiate these prepositions, the paper puts forward a pragma-semantic description according to which expressions with à, followed by an infinitival complement, are used when the speaker’s intention is to present a fact in an “ambivalent” way, whereas expressions with de indicate a “monovalent” vision of a fact. The study also tries to show that the “ambivalent vision” related to the preposition à makes it possible to introduce a particular argumentative value to the statement, indicating a real or a potential opposition of points of view.