Critique and denial of metaphysical beliefs in God or divine beings. Unlike agnosticism, which leaves open the question of whether there is a God, atheism is a positive denial. It is rooted in an array of philosophical systems. Ancient Greek philosophers such as Democritus and Epicurus argued for it in the context of materialism. In the 18th century David Hume and Immanuel Kant, though not atheists, argued against traditional proofs for God's existence, making belief a matter of faith alone. Atheists such as Ludwig Feuerbach held that God was a projection of human ideals and that recognizing this fiction made self-realization possible. Marxism exemplified modern materialism. Beginning with Friedrich Nietzsche, existentialist atheism proclaimed the death of God and the human freedom to determine value and meaning. Logical positivism holds that propositions concerning the existence or nonexistence of God are nonsensical or meaningless.