For context, Sam Vimes is musing inwardly about his younger self, whom he suddenly needs to mentor in lieu of his (own) original one due to a fantasy world-based magical freak accident in space and time (Night Watch, p. 163):
“He wondered if it was at all possible to give this idiot some lessons in basic politics. That was always the dream, wasn’t it? ‘I wish I knew then what I know now’? But when you got older, you found out that you now wasn’t you then. You then was a twerp. You then was what you had to be to start out on the rocky road of becoming you now, and one of the rocky patches on that road was being a twerp.
A much better dream, one that’d ensure sounder sleep, was not to know now what you didn’t know then.”