Metaphysical naturalism refers to a belief about the totality of what exists. It is a belief that nature is all that exists and assumes that observable events in nature are explained only by natural causes. It entails that all concepts related to consciousness or to the mind refer to entities which are reducible to or supervene on natural things, forces and causes. More specifically metaphysical naturalism rejects the objective existence of any supernatural thing, force or cause, such as are described in humanity’s various religions and mythological accounts.
Most metaphysical naturalists agree that the fundamental constituents of reality, from which everything derives and upon which everything depends, are fundamentally mindless. So if any variety of metaphysical naturalism is true, then any mental properties that exist are causally derived from systems of nonmental properties, powers, or things.
Humanity’s existence as conscious and intelligent animals, is explained not as the outcome of intelligent design nor as a mere accidental combination of chemicals (such as originated life), but as the product of a dynamic, random system that generates highly complex order on its own, without any guidance. Since this entails that the properties of living organisms have been derived from random generation of diversity, genetic drift and natural selection, naturalists interpret individual organisms and species as not having any theleological purpose whatsoever, moral or otherwise, as nature is the cause and nature has no plan.
One major way in which naturalism explains things better than alternatives is that if the supernatural exists, whether as gods, powers, or spirits, it is so silent and inert that its effects are almost never observed, despite vast and extensive searching. Even the relatively few alleged observations take place only under dubious conditions lacking in sound empirical controls or tests, and on those occasions when they are subsequently subjected to sound controls or tests, they turn out to be false. Our inability to uncover clear evidence of anything supernatural is somewhat improbable if anything supernatural exists, but very probable if nothing supernatural exists, and therefore metaphysical naturalism is probably true.