If the bond is enough to justify, it also excuses the clan against the outsider.

Objects to

The impulse that makes me put my own first feeds, on a larger scale, nepotism, the clan spirit, the closing of the door to the outsider.
If the bond is by itself a sufficient reason, what stops me from preferring my tribe to humanity, and shutting my door on whoever does not belong to it?
Many injustices come not from neglecting one's own, but from the opposite excess: loyalty to one's own raised into the only law.

Therefore A partiality with no counterweight thus opens the way to the most unjust preferences, for lack of a principle to bound it.

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