Naturalism is the first assumption of ontological philosophy. It is the belief that the world is just the natural world, meaning the world that is disclosed to us by perception.

Since the role of naturalism in ontological philosophy is to identify what needs to be explained, it is appropriate to define naturalism ontologically, that is, in terms of what it says exists.

Positively, what exists, according to naturalism, is everything in space and time, for that is the world disclosed to us by perception. It is a world of objects in space and time that move and interact.

Negatively, naturalism denies the existence of anything that is not located in space and time, that is, anything that is supposed to be outside of either space or time (or both).

Naturalism denies the existence of God, if God is supposed to exist outside of space and time, as would be required for God to be the creator of the natural world.

Naturalism denies the existence of Platonic Forms, for they are supposed to exist in a realm of Being separate from Becoming (or the natural world). More generally, it denies the existence of abstract entities of any kind, it they are supposed to be entities that do not exist in space and time.

Naturalism denies the existence of minds, if they are entities that are supposed to exist outside space, as Descartes believed.

Though naturalism must deny that anything exists outside space or time, it does not deny the phenomena that lead people to believe in such entities. On the contrary, to be adequate, naturalism must somehow explain all those phenomena.

Though naturalism must deny the existence of Cartesian minds, it must explain the existence of consciousness. That is, in addition to all the cognitive processes and motivations attributed to mind, it must explain why experience has a phenomenal appearance, or what it is like to be a rational subject.

Though naturalism must deny that there are Platonic Forms (or abstract entities of any kind), it must explain the phenomena that originally lead to the belief in such supernatural entities. They include not only the apparent rational intuition of abstract entities, but also the appearance that there is real difference between good and bad in the world (that is, a difference that is not merely a projection of our feelings onto objects in the world).

Though naturalism must deny the existence of a transcendent God, it must explain the phenomena that lead to belief in God. To be adequate, naturalism must explain the appearance that there is something worthy of worship in the world. And there would be no doubt about its adequacy, if it explained God in a way that confirms the appropriateness of the religious attitude.