Several legal arguments use the notion of ratio legis in order to sustain a normative conclusion, in particular the argument from analogy and some forms of teleological argumentation. However, determining the ratio is often a difficult and controversial task. In this paper we look firstly at the speech acts typically performed by legal practitioners in order to determine the ratio and, secondly, we take into account the argumentative commitments they undertake in so doing and the argumentative constraints put on them. (...) This will lead us to distinguish different uses of the notion of ratio legis and to ask on what conditions each of them is justified. (shrink)