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You cannot say "thank you" in Sanskrit. It would be ridiculous to deduce from this (as William Ward, a British Orientalist, did in 1822) that gratefulness as a sentiment was unknown to the ancient Indian people. It is no less ridiculous to argue that rationality as a concept is absent from or marginal to the entire panoply of classical Indian philosophical traditions on the basis of the fact that there is no exact Sanskrit equivalent of that word.