It is in Chapter Eighteen when Jaggers finds Pip and Joe in the Three Jolly Bargemen and confronts Pip with his Great Expectations. This is, of course, what Pip has always wanted since he met Estella for the first time and became so aware of his humble origins which he is now ashamed of. However, before he can take up his great expectations, he must agree to two stipulations, which Jaggers presents in his customary style.
The first condition is that Pip must always bear the name of Pip. Secondly, the name of Pip's benefactor must remain secret until this mystery person chooses to reveal his or herself to Pip. Herein lies the problem that Pip falls into, because he assumes, as do we as readers, that his benefactor is Miss Havisham, even though there is no proof given. The fact that Pip is told he is not aloud to try and find out the identity of his benefactor makes his curiosity that much greater:
"Now you are distinctly to understand that you are most positively prohibited from making any inquiry on this head, or any allusion of reference, however distant, to any individual whomsoever as the individual, in all the communications you may have with me."
Thus the stage is set for the massive shock and surprise that Pip and we experience when he realises that it is actually Magwitch who is his benefactor and not Miss Havisham.