[SI_Rob]

I think you might be unfamiliar with the term "dualism" since you don't appear to understand that your reasoning is synonymous with current spec, put briefly: "In general, [Dualism] the idea is that, for some particular domain, there are two fundamental kinds or categories of things or principles." [0]

To what domain, exactly, are you trying to relegate concepts with your vaguely dismissive take that they are "inside people's heads," as if this doesn't invalidate thought as an origin of material (that is, natural) change and thus pulls the rug out from under your statement itself - since, being just a concept inside your head, it should not have been capable of accumulating the physical mass and energy necessary to get out of your head, onto HN's server, and onto my screen - the letters of which are not randomly generated but an ordered echo of material pointing back to the source informing their order.

The relationship of concept to material is, especially in this kind of case, about as intrinsic as a relationship between entities can be.

When you look at, say, a pyramid in Egypt or Mexico etc, you are looking at the material shadow cast by nothing less than a concept that was at some point, only "inside someone's head" and which remains a fundamental and intrinsic characteristic of that structured mass of earth and stone.

[0]https://plato.stanford.edu/entries/dualism/