Where does this capacity to be useful reside?
One question remains: within the human person, where exactly does the capacity to become useful to others reside? African oral traditions, carried across several Bantu languages and beyond, offer a remarkably consistent answer: this seat is the Heart/Conscience — a hub where intelligence, will and sensitivity are articulated, and which arbitrates between them to dictate the right course of action. To reflect, on the Way of We, is not to coldly calculate self-interest, but to question one's Heart/Conscience.
This is neither a neuroanatomical hypothesis nor a confessional entity: it is a concept of moral philosophy, of the same order as conscience in Rousseau or moral sense in Adam Smith. It allows concrete situations to be distinguished — the parent who carries a child, whose act fully falls under Useful-Existence even without immediate reciprocity, from the conscious adult stowaway who durably refuses to contribute.