The problem of evil is the question of how to reconcile the existence of evil and suffering with an omnipotent, omnibenevolent, and omniscient God. There are currently differing definitions of these concepts. The best known presentation of the problem is attributed to the Greek philosopher Epicurus. It was popularized by David Hume. Besides the philosophy of religion, the problem of evil is also important to the fields of theology and ethics. 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. Responses to the problem of evil have traditionally been in three types: refutations, defenses, and theodicies. The problem of evil is generally 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 a 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 a 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. Definitions Evil A broad concept of evil defines it as any and all pain and suffering, yet this definition quickly becomes problematic. Marcus Singer says that a usable definition of evil must be based on the knowledge that: "If something is really evil, it can't be necessary, and if it is really necessary, it can't be evil". According to John Kemp, evil cannot be correctly understood on "a simple hedonic scale on which pleasure appears as a plus, and pain as a minus". The National Institute of Medicine says pain is essential for survival: "Without pain, the world would be an impossibly dangerous place". While many of the arguments against an omni-God are based on the broadest definition of evil, "most contemporary philosophers interested in the nature of evil are primarily concerned with evil in a narrower sense". The narrow concept of evil involves moral condemnation, and is applicable only to moral agents capable of making independent decisions, and their actions; it allows for the existence of some pain and suffering without identifying it as evil. Christianity is based on "the salvific value of suffering". Philosopher