§29

In §28 Wittgenstein had made the point that ostensive definitions can be misinterpreted. In §29 he suggests that misinterpretations might be averted by saying 'this number is called 2" (as opposed to "this is 2". This has the advantage of giving the 'place in language' (i.e. locating it as a number word rather than a colour word or some other kind of word). However, Wittgenstein points out that in order for this to be effective the word 'number' must be defined first.

Wittgenstein grants that misunderstandings can be averted  in this way - but one still has to explain the meaning of the categorial terms. - And you explain them in terms of other words, which themselves need explaining. This sounds as if it will lead to an infinite regress or to circularity - either you'll end up defining terms in terms of other terms which you'd defined earlier in the chain or you'll end up continually explaining terms in terms of other terms.

Wittgenstein suggests, though, that it would be a mistake to think that the chain does not have an end. Just because you could go on explaining words in terms of other words does not mean that there is no end to the explanations. (Just because you could build another house at the end of a row of houses does not mean that the row of houses has no end).

Wittgenstein says that explanations can come to an end when you're satisfied that the other person has taken the definition as you wish them to - and they will demonstrate that in how they use the word.

I'm not sure that this is entirely satisfactory in itself. People start out with no language at all and they must have some way of acquiring it other than through linguistic explanations it seems. Is the answer that they begin to learn through ostensive training? I'm not sure.