Philosophers call this
the problem of other minds:
Two Zen monks walking through a garden stroll onto a small bridge over a goldfish pond and stop to lean their elbows on the railing and look contemplatively down at the fish. One monk turns to the other and says, “I wish I were a fish; they are so happy and content.” The second monk scoffs: “How do you know fish are happy? You’re not a fish!” The reply of the first monk: “Ah, but how do you know what I know, since you are not me?”
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Yoram Bauman, Quantum Microeconomics (p. 13)