This book is dedicated to the theory of knowledge and method of deduction of Abu'l-Hasan al-Ash'ari. Abu'l-Hasan al-Ash'ari, who tried to find a middle way between Ahl al-Hadith and Mu'tazila, contributed to the science of Kalam and reconciled disagreements between hadith and fiqh scholars and theologians. After introducing his life, works, the formation of his school, and method, the author discusses various topics and themes of al-Ash'ari's thought, including ontology, the attributes of Allah, morality, justice, guidance-misguidance, and imamate. The author contextualizes his method with other important names dealing with knowledge, like Plato, Aristotle, Locke, Descartes, Kant, and Bacon.