Ilm al-kalam or ilm al-lahut, often shortened to kalam, is the scholastic, speculative, or rational study of Islamic theology (aqida). It can also be defined as the science that studies the fundamental doctrines of Islamic faith (usul al-din), proving their validity, or refuting doubts regarding them rationally via logic. Kalām was born out of the need to establish and defend the tenets of Islam against philosophical doubters and non-Muslims, and also to defend against heretical and religious innovations (bidʿah). A scholar of kalam is referred to as a mutakallim (plural mutakallimun), a role distinguished from those of Islamic philosophers and jurists.
After its first beginnings in the late Umayyad period, the Kalām experienced its rise in the early Abbasid period, when the Caliph al-Mahdi commissioned Mutakallimūn to write books against the followers of Iranian religions, and the Barmakid vizier Yahya ibn Khalid held Kalām discussions with members of various religions and confessional groups in his house. By the 10th century, the Muʿtazilites were main pioneers of 'Kalam' during the early formative period of Islam. However, due to increased criticism by traditionalist Muslim scholars that the Mu'tazilites started departing from mainstream Sunni orthodoxy, they were refuted heavily. Soon after, two new important Sunni Kalām schools emerged: the Ashʿaris and the Maturidis. They positioned themselves against the growing Neoplatonic and Aristotelian philosophy within the Mu'tazilites and elevated the "Kalām science" (ʿilm al-kalām) as an acceptable ranking science in mainstream Sunni discourse. Some of the arguments of these Mutakallimūn also found their way into Jewish and Christian theological discussions in the Middle Ages. Kalām science by the early modern period was essentially limited to the study of manuals and commentaries, from the late 19th century onwards various reform thinkers appeared in British India and the Ottoman Empire who called for the founding of a "new Kalām".
