You're not a fish, so how do you know it's happy?

Watching fish swim in a river, Zhuangzi remarked on their happiness. His friend Hui Shi objected: you aren't a fish, so you can't know what fish feel. Zhuangzi replied that Hui Shi wasn't him either, so by his own logic he couldn't know what Zhuangzi did or didn't know — turning the challenge back on the sceptic.

— Zhuangzi (Zhuang Zhou), Zhuangzi — Autumn Floods chapter, c. 4th century BCE