§179

Wittgenstein now returns to the case from §151. This was the case of A writing down a series of numbers while B observes. B then says 'Now I can go on!' when he understands (or thinks he understands) how the series is to be continued.

The formula occurring to B is not sufficient for understanding. In order for him to have the right to say 'now I know how to go on!' experience must show him that there was a connection between the formula occurring to him and him actually continuing the series.

Wittgenstein again emphasises that the circumstances play a role in whether B used the words 'now I know how to go on' correctly ('circumstances' meaning things like - that B had learnt algebra). What exactly is the relationship between the circumstances and B being justified in saying 'now I know how to go on (continue the series)'? - Circumstances cannot be sufficient for justification - it is surely possible for B to have learnt algebra, for a formula to have occurred to him, for him to have said 'now I know how to go on' buit yet still to find that B wasn't able to continue correctly (that B hadn't understood).

Wittgenstein also suggests that we think about the circumstances in which expressions like 'now I know how to go on' are learnt and where they are used.