How many verbal patterns are there in Hebrew? From Sibawayh to Judah Hayyuj and Abraham ibn Ezra

In the introduction to his dictionary, considered to be the first scientific Hebrew dictionary based on the principle of the triconsonantal root, medieval grammarian Judah Hayyuj enumerates the conventional verbal patterns presently accepted, and in addition po'el. However, Hayyuj's identification of this latter pattern was not accepted by all medieval grammarians. Abraham ibn Ezra in particular disputed the existence of this pattern among the strong verbs. This article suggests that the difference between Hayyuj's and ibn Ezra's views is a result of their different historical contexts. In the tenth and eleventh centuries, the science of Hebrew linguistics, and especially grammar, followed the Arabic model. Accordingly, Hayyuj created the pattern po'el as the parallel to fa'ala, one of the four basic forms of the Arabic verb. By the twelfth century, however, ibn Ezra revaluated the issue using the data of Hebrew itself, and found no evidence for this form.