This book aims to shed light on the reasoning of a 10th-century grammarian, Ibn ?inni, based on his major work: al-?a?a?i?. He studies the way Ibn ?inni seeks to systematize grammatical explanatory reasoning and attempts to highlight the diachronic approach to the Arabic language adopted by Ibn ?inni in order to explain anomalies (sawa??) in the language system, resorting in particular to the notions of qiyas (analogy) and isti?mal (usage).