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.