The problem of evil is the question of how to reconcile the existence of evil and suffering with an omnipotent, omnibenevolent, and omniscient God. The best known presentation is attributed to the Greek philosopher Epicurus by David Hume, who was responsible for popularizing it. Hume summarizes Epicurus's version of the problem as follows: "Is God willing to prevent evil, but not able? Then he is not omnipotent. Is he able, but not willing? Then he is malevolent. Is he both able and willing? Then from whence comes evil?"
Responses to the problem have traditionally been discussed under the heading of theodicy. Besides philosophy of religion, the problem of evil is also important to the field of theology and ethics.
The problem of evil is often formulated in two forms: the logical problem of evil and the evidential problem of evil. The logical form of the argument tries to show a logical impossibility in the coexistence of God and evil, while the evidential form tries to show that given the evil in the world, it is improbable that there is an omnipotent, omniscient, and wholly good God. The problem of evil has been extended to non-human life forms, to include animal suffering from natural evils and human cruelty against them.
Responses to various versions of the problem of evil, meanwhile, come in three forms: refutations, defenses, and theodicies. A wide range of responses have been made against these arguments. There are also many discussions of evil and associated problems in other philosophical fields, such as secular ethics, and evolutionary ethics. But as usually understood, the "problem of evil" is posed in a theological context.
The problem of evil acutely applies to monotheistic religions such as Christianity, Islam, and Judaism that believe in a monotheistic God who is omnipotent, omniscient and omnibenevolent; but the question of "why does evil exist?" has also been studied in religions that are non-theistic or polytheistic, such as Buddhism, Hinduism, and Jainism.