But if you follow that logic further, which you should, it's also true that part of that capital is "human capital", or what some economists called variable cap

But if you follow that logic further, which you should, it's also true that part of that capital is "human capital", or what some economists called variable capital. I can't think of any increase in productivity which has not been the result of human labour (whether technical, scientific, in discovery of new natural materials, or otherwise). Non-labour ("constant") capital is inert (literally, as physical goods, it just sits in a warehouse) without labourers to work on or with it.