In the present paper I argue that it is possible to find a meaningful answer to the question why law should not be considered a true conditional particle. Although law is mentioned by early Arab grammarians within chapters or sections dedicated to conditional particles, only those words which are construed with the apocopate are regarded by the Arabic grammatical tradition as proper ḥurūf al-sarṭ. By exploring Ibn al-Ḥājib’s concepts of muḍiyy and of istiqbāl—helped in this by al-ʾAstarābāḏī’s analysis—and by remaining at the same time firmly attached to Sībawayhi’s treatment of ʾin, I argue that Arab grammarians were at least as much conscious as the Greek grammarians were about the fact that the semantic difference was much more important than the syntactic one. The syntactic constraint against law governing an apocopate in fact implies that the expression introduced by law is either impossible or necessary, and thus in both cases certain and non-hypothetical.