“Only after learning do we know what we lack; only after teaching do we know what is difficult.” The sentence comes from the Xue Ji chapter of the Book of Rites. The preceding lines compare the finest food, whose taste is unknown until eaten, with a worthy way that is unknown until studied. The text then says that recognising insufficiency enables self-examination, while encountering difficulty in teaching prompts further effort—hence teaching and learning advance one another.
It does not claim that teaching automatically produces mastery. Learning and teaching are two tests: the first exposes what we do not yet understand; the second forces vague understanding to meet another person’s questions. When an explanation fails, the listener may not be the only source of difficulty. A boundary in our own knowledge may finally have become visible.
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