The concept of substance, not any specific value given to that variable in this case.
I consider idealism primarily a methodological choice, because it's much less limiting than materialism. And materialists make their methodological choice for the same reason in inverse, they want the limiting aspect. The technologial teleology of mechanical repeatability requires the limiting aspect. In the holistic look, our ability to build machines and mechanize functions, is of course part of mental causation.
BTW, do you ever wonder how and why push out mineral goo out of our earlobes? Not the vax stuff in the ear channel, the lobe stuff that I just scratched plenty under my nail.