When Pip's chin is grabbed by Magwitch, it is the first time he becomes aware of himself as a person, with a life and with the potential to die. This awareness is caused by "a man who had been soaked in water, and smothered in mud," by a creature who comes froms the depths of the fens. Although he does not know it at the time, this is the beginning of Pip's expectations-- an event that will lead him into the adventures and mishaps that shape his life.
Swift and Dickens relate the course of a person's interactions with other people to the course of a person's interaction with nature. Both Mary and Pip become aware of themselves in an unexpected brush with nature. Although Magwitch is a human, he is described as an animal, and even he himself cries, "I wish I was a frog. Or a eel!"
Epiphanies can be slimy.