Judah ben David Hayyuj

He was the first to propose that Hebrew words stem from triconsonantal roots.

Hayyuj was a pupil of Menahem ben Saruq, whom he later helped to defend against the attacks of Dunash ben Labrat and his followers. Later in life, Hayyuj developed his theories about Hebrew grammar and was obliged to step forward as an opponent of the grammatical theories of his teacher. His thorough knowledge of Arabic grammatical literature led him to apply the theories elaborated by Arabic grammarians to Hebrew grammar and thus become the founder of the scientific study of that discipline. Preceding scholars had found the greatest difficulty in accounting, by the laws of Hebrew morphology, for the divergences existing between the so-called "strong" and "weak" verbs. Much ingenuity was spent discovering the principles that controlled the conjugation of the verbs. The weakness of Menahem's assertion that there are stems in Hebrew containing three letters, two letters, and one letter, respectively, was pointed out by Dunash; but, although the latter was on the road to a solution of the problem, it was left to Ḥayyuj to find the key.

Hayyuj announced that all Hebrew stems consist of three letters and maintained that when one of those letters was a "vowel letter," such a letter could be regarded as "concealed" in diverse ways in the various verbal forms. To substantiate his theory, he wrote the treatise upon which his reputation chiefly rests, the Kitāb al-Af‘āl Dhawāt Ḥurūf al-Līn "The Book of Verbs Containing Weak Letters." The treatise is in three parts: the first is devoted to verbs whose first radical is a weak letter, the second to verbs whose second radical is weak, and the third to verbs whose third radical is weak. Within each division, he furnishes what he considers a complete list of the class' verbs, enumerates various verb forms, and, when necessary, adds brief comments and explanations. Preceding each division, the principles underlying the formation of the stems belonging to the division are systematically outlined in a series of introductory chapters. As a supplement to this treatise he wrote a second, which he called the Kitāb al-Af‘āl Dhawāt al-Mithlayn "The Book of Verbs Containing Double Letters", and in which he points out the principles governing the verbs whose second and third radicals are alike. He furnishes a list of these verbs and their various forms occurring in the Bible. Besides the two treatises on verbs, Hayyuj wrote Kitāb al-Tanqīṭ "The Book of Punctuation". This work, probably written before his two chief treatises, attempts to set forth the features underlying the Masoretic use of the vowels and tone. In this work, he deals chiefly with nouns, and their purpose is more of a practical than of a theoretical character. A fourth work, the Kitāb al-Nutaf "The Book of Extracts," is known to have been written by Hayyuj, but only a few fragments and quotations by later authors have survived. This was a supplement to his two grammatical works on the verb, and he noted the verbs he omitted in the former treatises. In doing this he anticipated in a measure Jonah ibn Janāḥ's Mustalḥaq, which was devoted to this very purpose. He arranged and discussed the verbal stems in question, not alphabetically, but in the order in which they occur in the Hebrew Bible. An edition consisting of all discovered fragments was published by Nasir Basal in 2001.

The authorities additionally record the heading Abu Zakariyya Yahya ibn Dawūd Hayyūj. His recorded languages were Arabic and Hebrew. His recorded confession was Judaism.