the world Widely held historical and scientific opinion Plausible moral principles As a response to the problem of evil, a theodicy is distinct from a defence. A defence attempts to demonstrate that the occurrence of evil does not contradict God's existence, but it does not propose that rational beings are able to understand why God permits evil. A theodicy shows that it is reasonable to believe in God despite evidence of evil in the world and offers a framework which can account for why evil exists. A theodicy is often based on a prior natural theology, which exist to prove the existence of God, and seeks to demonstrate that God's existence remains probable after the problem of evil is posed by giving a justification for God's permitting evil to happen. Defenses propose solutions to the problem of evil, while theodicies attempt to answer the problem. Pseudo-Dionysus defines evil by those aspects that show an absence of good. Writers in this tradition saw things as belonging to 'forms' and evil as an absence of being a good example of their form: as a deficit of goodness where goodness ought to have been present. In this same line of thinking, St. Augustine also defined evil as an absence of good, as did theologian and monk Thomas Aquinas, who stated that "a man is called bad insofar as he lacks a virtue, and an eye is called bad insofar as it lacks the power of sight." Bad as an absence of good resurfaces in Hegel, Heidegger and Barth. Very similar are the Neoplatonists, such as Plotinus and contemporary philosopher Denis O'Brien, who say evil is a privation. Marxism, "selectively elaborating Hegel", defines evil in terms of its effect. Philosopher John Kekes says the effect of evil must include actual harm that "interferes with the functioning of a person as a full-fledged agent". Christian philosophers and theologians such as Richard Swinburne and N. T. Wright also define evil in terms of effect, stating that an "act is objectively good (or bad) if it is good (or bad) in its consequences". Hinduism defines evil in terms of its effect saying "the evils that afflict people (and indeed animals) in the present life are the effects of wrongs committed in a previous life." Some contemporary philosophers argue a focus on the effects of evil is inadequate as a definition since evil can observe without actively causing