The judgement of apostasy on the last page of al-Ghazali’s Incoherence of the Philosophers has long been considered as one of the key reasons for the decline of peripatetic philiosophy in Islam. This study is an inquiry into the different elements of Islamic thought which led to al-Ghazali’s conviction that three important conclusions of peripatetic philosophy must be considered as heresy. In its four parts, the volume describes the early usage of the judgement on apostates in Islam and the legal and theological developments, which led to its application against the secret apostates. It examines, how during the last decade of the 5th/11th century, the judgement could be turned against the philosophers. Finally the study determines the early effects of al-Ghazlai’s judgement on the peripatetic philosophy both in the Mashriq and in al-Andalus.