The main entry notes in passing that Ash'ari theology holds reason cannot determine good and evil independently of revelation while Mu'tazila theology holds the opposite, but this compressed summary understates how directly the two schools fought out a version of the Euthyphro dilemma centuries before it became a live topic in Western philosophy of religion.
The Mu'tazila, the rationalist school dominant in ninth-century Abbasid court theology, held that good and evil are objective properties of actions, knowable by unaided reason, so God commands what is good because it is good, exactly the "intellectualist" position the main entry associates with Aquinas; on this view God's commands are necessarily good because God, being just by nature, could not command otherwise, a position the Mu'tazila connected to divine justice ('adl), one of their five foundational principles. The Ash'ari school, which became dominant Sunni orthodoxy after the Mu'tazila's political eclipse, rejected this as placing a standard above God, holding instead that good and evil have no meaning prior to God's command: an act is good simply because God has commanded it, so if God commanded cruelty, per impossibile, it would thereby become good, a consequence Ash'ari theologians accepted rather than treated as a reductio.
The dispute had direct practical consequences: Mu'tazila legal theory could use reason to identify obligations where revelation was silent, while Ash'ari legal theory held nothing obligatory or meaningful until revelation specifies it, a difference shaping centuries of Islamic legal theory (usul al-fiqh) over how much weight reason carries in deriving law, giving the dispute a continuing institutional afterlife the parallel Christian debate never quite acquired.