  Al-Kindi uses philosophy to defend and explicate Islam in several works. He wrote a short treatise attacking the Christian doctrine of the Trinity, using concepts drawn from the Isagoge of Porphyry (al-Kindi’s refutation was the subject of a counter-refutation by the tenth century Christian philosopher, Yahya ibn ‘Adi; see Périer 1920). While this is the only extant work that engages in theological controversy, we know from the Fihrist that he wrote other treatises on similar themes. The extant corpus also contains passages in which al-Kindi expounds the meaning of passages from the Koran. Most striking, perhaps, is his discussion of creation ex nihilo in the midst of On the Quantity of Aristotle’s Books. This passage is a commentary on Koran 36: 79–82. Al-Kindi mentions the same Koranic passage, and discusses the special nature of prophetic knowledge, in a meteorological work entitled On Why the Higher Atmosphere is Cold (see Abu Rida 1953, 92–93). The cosmological work On the Prostration of the Outermost Sphere, meanwhile, is entirely devoted to explaining the Koranic verse “the stars and the trees prostrate themselves” (55: 6) in terms of al-Kindi’s account of heavenly influence on the sublunary world. Al-Kindi’s remarks here show his interest in the contemporary disciplines of grammar and Koranic exegesis.