Good and Evil Are Knowable by Reason

Mutazili theology, inherited in this respect by Zaydi, Twelver and Ismaili thought, holds that good and evil are objective moral qualities that human reason is capable of recognising on its own, independently of what revelation happens to say about them. On this account a person could, in principle, come to know that honesty is good and cruelty is evil through unaided reasoning, prior to and apart from any scripture instructing them so. Revelation's role is accordingly not to constitute morality from nothing, inventing categories of good and evil that would otherwise not exist, but to confirm and complete a moral order reason can already partly access, filling in details reason alone could not reach, such as the specific content of ritual obligation, while leaving the basic moral categories themselves within reason's own competence. This is a substantive and contested epistemological claim about where moral knowledge comes from, not merely an observation that scripture and common sense usually happen to agree. The Mutazilah gave this position a technical name, husn wa qubh aqli, the rational goodness and badness of acts, distinguishing it from the Ashari counterposition that an act's goodness or badness is established only by revelation and has no standing independent of what God commands or forbids.

The doctrine that reason can know good and evil independently of revelation is not an isolated position within Mutazili thought; it is what makes the parallel doctrine of adl, divine justice, a claim with actual content rather than an empty label. If good and evil had no standard except whatever God happens to command, then calling God just would say nothing beyond that God does what he does, since justice would simply be defined as divine action rather than measured against it. Only if reason has independent access to a real standard of good and evil can the statement that God is just function as a genuine claim, one that could in principle be false, rather than a tautology true by definition. This is why the two Mutazili principles travelled together into Zaydi, Twelver and Ismaili theology as a linked pair rather than as separate, independent commitments: divine justice requires an external standard for justice to be measured against, and the doctrine that good and evil are knowable by reason is precisely what supplies that standard. The same pairing put Mutazili theology at odds with the later Ashari doctrine of kasb, acquisition, which held that God originates every human act and a person merely acquires it, a position Mutazili thinkers rejected as incompatible with the genuine authorship adl requires.