You are not a fish, how do you know the fish is happy
A 4th-century-BC argument on a bridge never got resolved. Its unresolved shape is the whole point.
Zhuangzi and his friend Huizi were walking along a bridge over the Hao River when Zhuangzi looked down at the fish and said they looked happy, swimming freely. Huizi, a logician by temperament, pounced: you are not a fish — how could you possibly know what a fish feels?
It's a good question, and it's still basically unanswered two and a half thousand years later — it's the ancient Chinese ancestor of every modern argument about whether we can know another mind's experience from outside it. Zhuangzi's reply, though, is the part worth keeping. He didn't try to prove he knew the fish's inner state. He turned the logic back on Huizi: you are not me — how do you know that I don't know what the fish feels? If not being a fish disqualifies me from knowing fish-happiness, then not being me disqualifies you from knowing what I can and can't know.
The exchange usually gets read as a draw, or as Zhuangzi cleverly dodging a fair challenge with wordplay. But there's a less cynical reading: Huizi's question assumes that knowledge requires being identical to the thing known, which would make all empathy, all fellow-feeling, all reading of another creature's state technically illegitimate. Zhuangzi's response isn't a dodge. It's a refusal to accept that standard at all.
We still argue about whether you can "really" know what your dog wants, what your friend is feeling, what a stranger is thinking. The bridge conversation never settled it. It just showed you can keep walking without settling it.