What 'special agreement' does Pip make with Joe regarding Joe's education?

I believe the agreement he makes, although I don't think it is stated as such, is that he will continue to learn to read and write. Pip feels ignorant, and that he hasn't learned anything....... Joe on the other hand, is completely impressed that he can write. Thus, Pip agrees to continue on......... this can be found in Chapter 9;

`Well, Pip,' said Joe, `be it so or be it son't, you must be a common scholar afore you can be a oncommon one, I should hope! The king upon his throne, with his crown upon his 'ed, can't sit and write his acts of Parliament in print, without having begun, when he were a unpromoted Prince, with the alphabet -- Ah!' added Joe, with a shake of the head that was full of meaning, `and begun at A too, and worked his way to Z. And I know what that is to do, though I can't say I've exactly done it.'

There was some hope in this piece of wisdom, and it rather encouraged me.

`Whether common ones as to callings and earnings,' pursued Joe, reflectively, `mightn't be the better of continuing for to keep company with common ones, instead of going out to play with oncommon ones -- which reminds me to ho pe that there were a flag, perhaps?'