Apart from the exposition and interpretation of Quranic teaching, which is a vast subject, the critical problems that have preoccupied students of the Qur'ān fall for the most part into four divisions. First there is the question of the origins of the Qur'an in the broadest sense. How did the substance of the book come to be and what sort of material does it constitute? From what kind of roots in the social situation or the personal life of Muhammad does it spring? What is the relation to the literary deposits of other preceding religious traditions? It may be argued that this problem might more properly be considered as an aspect of study of the Prophet since it has to do with the modes of revelation, the nature of prophetic psychology and experience, and the possible prophetic acquaintance with or dependence upon religious writings of Christians and Jews, whether directly or through informants.