Philosophical naturalism

Philosophical naturalism is the doctrine that the natural world is all there is — in other words, that the supernatural is by definition impossible, since whatever is shown to exist (via affecting the natural world) is clearly part of that same natural world.

Basically all scientists (and a vast majority of modern philosophers) adhere to some version of naturalism, which is why reference to supernatural or miraculous effects or forces is frowned upon in academia.

Metaphysical or methodological?
Two refinements of the term "naturalism" that are of use for any rationalist to know are methodological naturalism (on the one hand) and metaphysical naturalism (on the other).

Science is itself a process based on methodological naturalism, i.e. treating the world as if metaphysical naturalism was the case, but without actually taking a stand on matters philosophical (outside of method).

Metaphysical naturalism, on the other hand, is what it means to extend scientific principles all the way into the farthest reaches of philosophy. Scouting future territory for the natural sciences to conduct research in (and defending naturalism generally), metaphysical naturalism is what Richard Carrier simply describes as "science with less data", operating in anticipation of scientific data where possible. This approach signifies the single least woo-prone way in which to engage any given speculatory topic, aside from the philosophically agnostic view of methodological naturalism.

Metaphysical naturalism is more or less a basic precept of all modern, which was itself formed partially in protest of the centuries of woo that had accreted in philosophy (notably German idealism) prior to the explosive triumph of modern science in the last 150 years or so.

Strict adherence to methodological naturalism creates a theoretical space in which positions like NOMA may be compatibly held by those who chose to do so. Methodological naturalism is, as such, the "minimum position" to which science is reducible — and it still entails not giving the time of day to woo-meistry.

In that context, metaphysical naturalism simply means "closing" said NOMA/gaps space by taking the fight all the way against theology and accommodationism.

Origins in methodological naturalism
Philosophical naturalism is arguably the logical end-result of methodological naturalism. Methodological naturalism accepts that there is no way to contact, detect, or otherwise empirically observe the supernatural. In consequence, methodological naturalists regard the scientific method as the best way to determine the truth about reality. Because supernatural, intelligent forces - if they exist - are unpredictable and hence unrepeatable, naturalists reject the possibility of supernatural or magical intervention in the physical world.

Philosophical naturalists take these beliefs one step further and reject the existence of the supernatural altogether, citing the utter lack of empirical evidence. Because of the absence of scientific evidence backing up religion, most philosophical naturalists are also atheists (or at most deists).

Opposition by religion
Arrayed against philosophical naturalism are religious schools of thought which argue for the existence of the supernatural alongside the natural. In order to compete with science, they have created pseudoscientific theories such as intelligent design which supposedly prove supernatural intervention in natural phenomena. However, they also assert that the supernatural is somehow immune to science. These theories, therefore, work about as well as cargo cults.

Do not equate with
Although these are related to naturalism, there are differences:

Materialism
All things are composed of matter and that all emergent phenomena (such as thoughts) are the result of material interactions.

Physicalism
The only things are those subject to the laws of physics.

Scientism
All that can be known is what science tells us.

Nominalism
Abstract objects exist as predicates, not universals (as opposed to Platonism).