Process theism is a twentieth-century school of theological thought that offers a nonclassical understanding of the relationship between God and the world. Classical Christian theists maintain that God created the world out of nothing and that God not only can, but does, unilaterally intervene in earthly affairs. Whitehead's pithy summary of process theism is "The many become one, and are increased by one". In Whitehead's system, only actual entities can have causal efficacy. The tension in the process conception of God between an eternal and unchanging evaluation of eternal objects and a temporal entity internally related to every other actuality has led to Whitehead's “dipolar” doctrine of God.