Michèle Monte deals with the use of si in non conditional dependant clauses. Based on a big corpus (press and essays), our research distinguishes three categories of si P clauses : topical clauses in which Q explains P, concessive ones in which Q limits P, and comparative ones where Q is compared with P. We explain these uses of si – which are not so marginal as it could seem – by the value of si in the system : we definite si as a morpheme that puts the allocutor in front of a choice he is invited to make although the context and the clause position already show the locutor's preference. In a second part we show how the dialogism works in each category : anticipating a counter-argument, bringing into use arguments belonging to the interdiscourse, resuming the locutor's speech (auto-reformulation). In a third part we insist on the argumentative interest of si in contexts where the locutor wants the interlocutor to think it is himself who validates the si P content. Thanks to its value in the system, si allows to benefit from elements belonging to the context while giving the idea their validation is still in doubt.