§184

Wittgenstein is attacking the reservoir model of the mind found in William James. When you have forgotten a tune and then you are suddenly able to sing it does this mean that the tune was there in your mind in some sense?
- Presumably Wittgenstein wants to deny that the tune is present in the mind in the sense in which water is stored up in a reservoir and can be released at some point.

It certainly wasn't the case that you heard the whole tune in your mind in a flash.
It could be - as in the case of the person who declares 'now I know how to go on' when observing a series being written down - that you think you know the tune but find you don't. - Having a feeling that you know is no guarantee that you do.
Hacker: "The 'presence of the tune to the mind' is not the ground of, and explanation of, the certainty or the exercise of the ability".