"There are three classes of friendship and enmity, since men are so disposed to one another either by preference or by need or by pleasure and pain."

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[I]t being reasonable and just, I should have a right to destroy that which threatens me with destruction: for by the fundamental law of nature, man being to be preserved as much as possible, when all cannot be preserved, the safety of the innocent is to be preferred: and one may destroy a man who makes war upon him, or has discovered an enmity to his being, for the same reason that he may kill a Wolf or a lion.

— John Locke