Filial piety (xiao) is the root from which Confucian ethics grows outward: learn deference and care at home, and the same capacities scale to teacher, ruler, and stranger. The scaling is the whole theory — you cannot love humanity while being contemptuous of your parents. Its costs have been argued about for centuries, since the same principle underwrote patriarchal authority and the subordination of the young and of women, and modern East Asian debates about the family run directly through it.