"Gratitude is a sickness suffered by dogs."

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Gratitude, as it were, is the moral memory of mankind. In this respect, it differs from faithfulness by being more practical and impulsive: although it may remain, of course, something purely internal, it may yet engender new actions. It is an ideal bridge which the soul comes across again and again, so to speak, and which, upon provocations too slight to throw a new bridge to the other person, it uses to come closer to him.

— Georg Simmel