Yeah it's difficult to know. When dealing with this kind of thing I try and take a scientific approach to it. Firstly by actually trying to define what we're actually talking about, and secondly seeing how we can gather evidence it exists or not. Believers in it don't seem to like this approach. They'd much rather speak in metaphors and rely on fallacies to "prove" it exists.
I've found some definitions, I'm not sure if they are "good" ones or not:
Materialism: Materialism is the view that the only thing that exists is matter; if anything else, such as mental events, exists, then it is reducible to matter. Matter here I think includes everything we can examine, so the structure of spacetime etc.
Naturalism: The system of thought holding that all phenomena can be explained in terms of natural causes and laws.
I don't think a scientific approach actually mandates either of these. It's just that so far the only successful* approach so far has been to assume things follow laws, and attempting to find out what these laws are. You get nowhere by just saying "things happen for unknown reasons, and we can't examine these reasons".
* Theists may differ on successful and argue circular reasoning, despite using a product of science to express this view.
I've found some definitions, I'm not sure if they are "good" ones or not:
Materialism: Materialism is the view that the only thing that exists is matter; if anything else, such as mental events, exists, then it is reducible to matter. Matter here I think includes everything we can examine, so the structure of spacetime etc.
Naturalism: The system of thought holding that all phenomena can be explained in terms of natural causes and laws.
I don't think a scientific approach actually mandates either of these. It's just that so far the only successful* approach so far has been to assume things follow laws, and attempting to find out what these laws are. You get nowhere by just saying "things happen for unknown reasons, and we can't examine these reasons".
* Theists may differ on successful and argue circular reasoning, despite using a product of science to express this view.
