 In his attempt to define the boundaries of Islam al-Ghazâlî singles out a limited number of teachings that in his opinion overstep the borders. In a separate book, The Decisive Criterion for Distinguishing Islam from Clandestine Unbelief (Faysal al-tafriqa bayna l-Islâm wa-l-zandaqa) he clarifies that only teachings that violate certain “fundamental doctrines” (usûl al-‘aqâ’id) should be deemed unbelief and apostasy. These doctrines are limited to three: monotheism, Muhammad’s prophecy, and the Qur’anic descriptions of life after death (al-Ghazâlî 1961, 195 = 2002, 112). He stresses that all other teachings, including those that are erroneous or even regarded as “religious innovations” (singl. bid’a), should be tolerated. Again other teachings may be correct, al-Ghazâlî adds, and despite their philosophical background, for instance, should be accepted by the Muslim community. Each teaching must be judged by itself, and if found sound and in accordance with revelation, should be adopted (al-Ghazâlî 1959a, 25–27 = 2000b, 67–70). This attitude leads to a widespread application of Aristotelian teachings in al-Ghazâlî’s works on Muslim theology and ethics.