Not the Onion

I think of it as the Marx materialist form of Hegel's externalized "geist" (spirit). He wrote

Man is directly a natural being. As a natural being and as a living natural being he is on the one hand endowed with natural powers, vital powers — he is an active natural being. These forces exist in him as tendencies and abilities — as instincts. On the other hand, as a natural, corporeal, sensuous objective being he is a suffering, conditioned and limited creature, like animals and plants. ... A being which does not have its nature outside itself is not a natural being, and plays no part in the system of nature. A being which has no object outside itself is not an objective being.

In short, it's part of human nature, or, as he called it "species-being" (Gattungswesen), as opposed to "species-life" (Gattungsleben), which is what all animals have. Non-human animals' lifeforce and life are one in the same, while humans direct their life force consciously. It's your will and drive, what you put into things, and eventually one of the things that are alienated from the worker by the capitalist system.