The true meaning of the phrase is that the humans are the ones that control and produce everything, and that without them, there's not a single things that can manage that. In this case we have the oil, as it is one of the most demanded resources, and one that makes enormous profit, but the oil can not extract itself, can not manufacture itself, and it can not use itself. For all those things, human action and brain power is needed, and the same goes for all the other things that make capital, as without the human capital, none of the other capitals would exist.