The Muʿtazilah is a religious creed of theological rationalism whose life traces back to the late first century AH, blossoming in the third decade of the second century AH, and by the fourth century AH it was the dominant doctrinal current in the Islamic community. From the fifth century AH onward, it began to decline, and in the seventh century AH it collapsed. With the dissolution of this sect, rationalism in the Islamic community was relegated to oblivion. In the last two centuries, a number of Islamic thinkers have defended Islam and refuted criticisms of the religion by employing Muʿtazilī methods, and they have come to be known as Neo-Muʿtazilah. This library-based, comparative analysis aims to clarify the points of difference and similarity between the two aforementioned currents. The two intellectual systems were convergent in foundational domains such as rationalism, philosophical orientation, defense and refutation of doubts about Islam, belief in the principles of faith, and so on; however, they diverged in interpretation of texts, underlying premises and implications of rationalism, the influence of Qur’anic text by the culture of its Revelation, the historicity of Islamic rulings, and related matters.