Full Idea
If someone says he only knows what pain is from his own case, suppose everyone had a box with something in it (a 'beetle'). We can't see into other boxes, so we say what a beetle is by looking in our own box. The contents of each box could be different.
Gist of Idea
If we only named pain by our own case, it would be like naming beetles by looking in a private box
Source
Ludwig Wittgenstein (Philosophical Investigations [1952], §293)
Book Reference
Wittgenstein,Ludwig: 'Philosophical Investigations', ed/tr. Anscombe,E. [Blackwell 1972], p.100
A Reaction
But pain is private. Children must guess the meaning of parts of the language that refer to inner experience, like 'sad' or 'hopeful'. Body language brings our private concepts together, but error seems possible.