Full Idea
Let's assume the child is a genius and itself invents a name for the sensation! - But then, of course, he couldn't make himself understood when he used the word.
Gist of Idea
If a brilliant child invented a name for a private sensation, it couldn't communicate it
Source
Ludwig Wittgenstein (Philosophical Investigations [1952], §257)
Book Reference
Wittgenstein,Ludwig: 'Philosophical Investigations', ed/tr. Anscombe,E. [Blackwell 1972], p.92
A Reaction
[His example is a sensation with no behaviour] Sensations are not just related to behaviour; they are related to external objects, and to parts of the body. We doubt the sensations of others if they can't name the object or the body part.