Dickens is a fan of word play, and he sometimes plays with idioms and paradoxes to show the intelligence (or unintelligence) of certain characters. For example, in chapter 43, Fagin riffs on two idioms to impress upon his new employee Noah Claypole the importance of looking out for Fagin's best interests:
We have a general number one; that is, you can’t consider yourself as number one without considering me too as the same, and all the other young people.
Fagin is playing with the idiomatic advice of looking out for "number one" (prioritizing one's own best interests above others' best interests). He is also playing with the idiomatic advice (drawn from the Bible): "do unto others as you would have them do unto you." These two maxims are contradictory, but he has merged them into a new golden rule for Noah and his other followers to abide by: look out for "number one" (yourself) by looking out for another "number one" (Fagin).
Fagin's advice here inverts the structure of a paradox. Both expressions baked into the advice are commonplace and true enough, so it sounds reasonable on the surface. Looking out for "number one" seems indeed to be the way London operates, and it's a familiar mindset to Noah, who has survived the Sowerberry household by looking out for himself. Thinking first of one's neighbors sounds like an even more virtuous code of conduct to strive for. Fagin thus uses language that makes him sound like a morally sound leader of a band of "number ones," and Noah at first accepts Fagin's golden rule at face value.
But as the conversation continues and the advice comes under more scrutiny, Noah begins to see that it does not make logical sense. Noah, under the pseudonym Bolter, corrects Fagin:
"Well, you can't take care of yourself, number one, without taking care of me, number one."
"Number two, you mean," said Mr Bolter, who was largely endowed with the quality of selfishness.
"No, I don't! retorted the Jew. I'm of the same importance to you as you are to yourself."
"I say," interrupted Mr Bolter, "yer a very nice man, and I'm very fond of yer; but we ain't quite so thick together as all that comes to."
Noah eventually sees through Fagin's manipulation. This falling apart of Fagin's logic echoes the narrator's verbal irony about the Poor Laws and the logic many "public servants" use when putting them into practice. For instance, the woman who takes care of Oliver in his infancy uses logic shot through with statistics to make a case for pocketing the money the state provides to feed the children. Dickens's novel uses these moments for comic relief, but it also consistently critiques those who use language to take advantage of others. The woman who steals the money instead of feeding Oliver is no better in this respect than Fagin, the villain. The novel thus teaches readers to think all the way through logical arguments before agreeing with them.