International audience ; According to Arabic grammar, a fa‑ must appear between the protasis (p) and the apodosis (q) of a hypothetical system in ʾin whenever the apodosis cannot be protasis, that is to say whenever the apodosis does not have the form faʿala (neutral perfect from the point of view of time) or yafʿal (imperfect apocopate). However, its presence is not only a formal constraint, it also meets a semantic necessity: systems ʾin p, q would mark a logical connection where q would be the logical consequent of p, its antecedent; systems ʾin p fa‑q would mark, on the other hand, a pragmatic connection, in other words everything else. The recognition, ancient by the medieval Arabic tradition and recent by the Arabist one, of two fundamental valuesof the fāʾ, causal (sababiyya) for the one, illative (taʿlīliyya) for the other, invites us, however, to glimpse that ʾin p fa‑q certainly harbours the pragmatic connection but also, for part, the logical one, but also that the distinction appears to be syntactically motivated, the fāʾ sababiyya preceding what is not an assertion, the fāʾ taʿlīliyya, on the other hand, anteceding precisely an assertion. This article then aims to show, based on a large number of examples, that this holds, both synchronically and diachronically, for if p fa‑q, where if stands for ʾin just as well as ʿiḏā and law. It will also show that when the apodosis is in the future tense, an interpretative ambiguity arises between causal and illative relations, and that the system, as it presents itself to us in all its diversity, is indeed a human material, and that we can therefore only deduce rules that are tendentially correct, but not definitive laws. ; D’après la grammaire arabe, un fa‑ doit apparaître entre la protase (p) et l’apodose (q) d’un système hypothétique en ʾin dès lors que l’apodose ne saurait être protase, c’est‑à‑dire dès lors que l’apodose n’a pas la forme faʿala (accompli neutre du point de vue du temps) ou yafʿal(inaccompli apocopé). Sa présence ne relève toutefois pas ...