It is not flesh and blood but the heart which makes us fathers and sons.

The saying contrasts biological kinship (“flesh and blood”) with chosen or earned bonds of affection (“the heart”). It suggests that the moral and emotional work of care, loyalty, and recognition is what truly constitutes family roles such as father and son, rather than mere genetic descent. In Schiller’s broader humanistic outlook, this aligns with an Enlightenment-inflected emphasis on inner character and ethical feeling as the basis of genuine social ties. The line is often invoked to validate adoptive, step-, or mentor relationships, and to argue that love and responsibility—not biology—create legitimate familial belonging.