(Arabic: “acquisition”), a doctrine in Islām adopted by the theologian al-Ashʿarī (d. 935) as a mean between predestination and free will. According to al-Ashʿarī, all actions, good and evil, are originated by God, but they are “acquired” (maksūb, whence kasb) by men. As for the criticism that his kasb theory attributes evil to God, al-Ashʿarī explained that, by creating evil, God is not an evildoer.

Al-Ashʿarī chose the term kasb to avoid attributing khalq (creation) to anyone but God. His main concern was to maintain God's total omnipotence and at the same time allow men a degree of responsibility for their actions. Al-Ashʿarī rejected the assertion of the Muʿtazilah theological school, of which he had been a member, that man has the power to will an act or its opposite. He maintained rather that man has the power to will only the act, not the opposite. Man does not initiate anything; he merely acquires what God has created. Thus man's responsibility comes from his decision as to which actions he should acquire.

Because of its limiting of man's scope and its emphasis on God's omnipotence, the kasb doctrine was regarded by many Muslim theologians as being indistinguishable from pure predetermination. Despite the efforts of al-Ashʿarī and his followers (the Ashʿarīyah) to clarify kasb, it remained one of the most vague theories in Islāmic theology, as the proverb aḍaqq min kasb al-Ashʿarī (“more subtle than the kasb of al-Ashʿarī”) indicates.