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Gratitude is a law of nature (the fourth).
As justice dependeth on antecedent covenant; so does gratitude depend on antecedent grace; that is to say, antecedent free gift; and is the fourth law of nature, which may be conceived in this form: that a man which receiveth benefit from another of mere grace endeavour that he which giveth it have no reasonable cause to repent him of his good will. For no man giveth but with intention of good to himself, because gift is voluntary; and of all voluntary acts, the object is to every man his own good; of which if men see they shall be frustrated, there will be no beginning of benevolence or trust, nor consequently of mutual help, nor of reconciliation of one man to another; and therefore they are to remain still in the condition of war, which is contrary to the first and fundamental law of nature which commandeth men to seek peace. The breach of this law is called ingratitude, and hath the same relation to grace that injustice hath to obligation by covenant.
Hobbes, Lev XV 16