The assumption of materialism

I am troubled that many science minded people assume materialism/physicalism; that everything is physical.

I agree that the scientific method provides provable knowledge about the physical universe, and that it is impossible to prove anything at all otherwise.

But proving something is true, and acknowledging its existence are two different things. For example, it is false that living physical unicorns roam the prairies on earth, but the idea of unicorns certainly exists in the minds of humans. Is an idea physical?

Certainly philosophical arguments and logic can prove whether arguments are true. And the realm of philosophical reflection has generated generations of generations of good ideas, often contradicting those of previous generations. But these are all mere ideas.

Newton's laws of motion are true within their proper domain, period. And this is the difference between science and philosophy (I'm using philosophy to refer to any idea-based reflection and analysis).

But there seems to be things outside of provable science. An example is a fictional novel. Science encompasses the ink on the paper, but science can say nothing about the story and the plot. To assume these are in some way physical is, in my view, a mistake.