In Chapter VII of Stage I of Great Expectations, Pip narrates,

Much of my unassisted self, and more by the help of Biddy than of Mr. Wopsle's great-aunt, I struggled through the alphabet as if it had been a bramble-bush; getting considerably worried and scratched by every letter. After that, I fell among those thieves, the nine figures, who seemed every evening to do something new to disguise themselves and baffle recognition. But, at last I began, in a purblind groping way, to read, write, and cipher, on the very smallest scale.

As a student of Mr. Wopsle's great aunt, Pip receives little formal education; instead, he must struggle through learning the alphabet on his own with some help from Biddy, the orphan girl who works for this great aunt. Thus, for Pip these exercises in practicing the alphabet are likened to his attempting passage through a bramblebush, slow and painful with many digressions. The numbers [figures] are personified as "thieves" because in his arithmetic problems, they seem to disguise themselves from their stated value by themselves. 

This imagery of brambles suggests the mental struggles of young Pip in a tangible way.  Everywhere Pip goes he feels confined and threatened.  At home he descends the stairs, hearing his conscience talk. At Satis House he is brought up dark stairs and put into a confining dark room. When Magwitch appears on his stairs at Barnard's Inn and talks with Pip, Pip "seemed to be suffocating" and he "shudders" at the face so close to him.

The personfication of the numbers as "thieves" contributes to the  prisoner imagery such as the leg-iron of Magwitch, the handcuffs, chains and the locked gates at Satis House, and the death masks in Mr. Jaggers's office.  This imagery of criminality and prison complements the theme of guilt in Great Expectations. 

In other passages, too, Dickens's narrator describes the thoughts and actions of people in such ways that the reader can picture them. For instance, Mr. Jaggers's efforts to remove his guilt are indicated by the images of his washing his hands. Joe's discomfiture when he visits Pip in London is suggested by the images of his hat being held "with both hands, like a bird's nest with eggs in it." Also, Joe's awkwardness is likened to a ruffled bird. Indeed, there is much imagery in Great Expectations, imagery that furthers the themes of society as a prison and the mental restrictions that Pip self-imposes.