A service, a sacrifice, a benefit, once accepted, may engender an inner relation which can never be eliminated completely, because gratitude is perhaps the only feeling which, under all circumstances, can be morally demanded and rendered. … If by itself or in response to some external reality, our inner life has made it impossible for us to continue

DEFINE: This passage defines gratitude as a unique moral relation — the one feeling universally demandable and renderable, establishing an inner connection that cannot be fully dissolved.

Georg Simmel, The Sociology of Georg Simm…, loc. 9389