This book is among the contributions that I offer to the British reader in particular, and to the Western reader in general, on Arab-Islamic philosophy. However, this book is distinguished from what I presented in my previous writings, as it is limited to one school of thought, excluding all other schools of thought, which is the doctrine of the Mutazilites. The Mutazilism scholars played a dangerous and important role in the second Hijri century, by cleaning the Arab mind of distortions and nonsense stuck in it, and their principle was “reason before quotation.” The intellect is the decisive factor towards the written texts and the spoken sayings, as they adhered to the verses of exaltation and gave the verses of analogy in accordance with the divine exaltation in the Qur’an. And so was their mental attitude towards the hadiths of the Prophet, so what the mind accepts they take, and what conflicts and contradicts it they reject and leave. Thus, the Mutazilites confronted some Islamic sects, led by the traditional conservatives of the Sunnites, who prioritize quotation over reason. Although the confrontation ended in favour of the traditional conservative approach, the Mutazilites were an infidel and fought against, their presence continued to be limited, scattered here and there. Also, their ideas, theories and some of their books continued to be preserved, whether in the libraries of the Islamic sects that adapted to them, such as the Zaydites in Yemen, or in the works written by their opponents and enemies, such as the Asharites, Hanbalites and others.