Logic

Logic was no less important for Islamic than for Christian philosophers and theologians. By the time of al-Farabi (c. 878–950), the first important Islamic logician, the whole of Aristotle's logical Organon was available in Arabic—far more material, then, than in the Latin West at this period, especially since the Arabic logicians tended to follow the habit of late antiquity in regarding the Rhetoric and the Poetics as parts of the Organon and assigning to them their own characteristic modes of argument, to contrast with demonstrative argument as taught in the Analytics and dialectical argument in the Topics.

Al-Farabi, who worked in Baghdad, Damascus, and elsewhere, saw his task as a logician to represent Aristotle faithfully, although this task involved him in a number of interpretations that went beyond the letter of the text. He was also concerned to vindicate logic in face of the grammarians, who doubted the need for this additional discipline; earlier in the tenth century, there had been a famous debate between the grammarian Abu Sa'id as-Sirafi and the logician Abu Bisr Matta, in which Abu Sa'id seems to have had the upper hand.

The great Persian philosopher Avicenna (Ibn Sina; 980–1037) respected al-Farabi and was also an ardent Aristotelian, but his approach to logic differed. In semantics, he rejected al-Farabi's theory that logic is concerned with expressions insofar as they signify meanings. Rather, he claimed, logic deals with meanings that classify meanings—so-called "second intentions." In his approach to syllogistic, Avicenna was far more inclined than al-Farabi had been to pursue his own train of analysis and accommodate Aristotle to it. For example, in modal logic he proposed a number of different readings of modal sentences: they could be taken absolutely (as in "God exists") or according to a condition—for example, "while something exists as a substance" (as in "man is necessarily a rational body") or "while something is described in the way it is" (as in "all mobile things are changing").

Logic continued to be studied in Islam because it was accepted by theologians as useful to their discipline rather than—as they often thought with regard to other areas of Aristotelian philosophy—a dangerous rival to it. Particularly important was the endorsement of al-Ghazali (1058–1111): whereas he wrote a work specifically designed to attack other areas of Aristotelian philosophy, he was himself the author of two short logical works, based on Avicenna. In the next century, Averroës (Ibn Rushd), who worked in Muslim Spain, followed al-Farabi (and the general direction of all his own work as the commentator par excellence on Aristotle) in seeking a greater fidelity to Aristotle, but it was Avicenna who remained the dominant influence on later Islamic logic.