Aristotle, Nicomachean Ethics
H. Rackham, Ed.

Also parents know their offspring with more certainty than children know their parentage; and progenitor is more attached to progeny than progeny to progenitor, since that which springs from a thing belongs to the thing from which it springs—for instance, a tooth or hair or what not to its owner—whereas the thing it springs from does not belong to it at all, or only in a less degree. The affection of the parent exceeds that of the child in duration also; parents love their children as soon as they are born, children their parents only when time has elapsed and they have acquired understanding,¹ or at least perception. [3] These considerations² also explain why parental affection is stronger in the mother. Parents then love their children as themselves （one's offspring being as it were another self—other because separate³）; children love their parents as the source of their being; brothers love each other as being from the same source, since the identity of their relations to that source identifies them with one another, which is why we speak of ‘being of the same blood’ or ‘of the same stock’ or the like; brothers are therefore in a manner the same being, though embodied in separate persons. [4] But friendship between brothers is also greatly fostered by their common upbringing and similarity of age; ‘two of an age agree,’⁴ and ‘familiarity breeds fellowship,’ which is why the friendship between brothers resembles that between members of a comradeship.