 In the version of the low priors argument formulated below, the basic approach described above is improved by comparing omni-theism, not simply to its denial, but instead to a more specific atheistic hypothesis called “source physicalism”. Unlike ontological physicalism, source physicalism is a claim about the source of mental entities, not about their nature. Source physicalists, whether they are ontological physicalists or ontological dualists, believe that the physical world existed before the mental world and caused the mental world to come into existence, which implies that all mental entities are causally dependent on physical entities. Further, even if they are ontological dualists, source physicalists need not claim that mental entities never cause physical entities or other mental entities, but they must claim that there would be no mental entities were it not for the prior existence (and causal powers) of one or more physical entities. The argument proceeds as follows: