In one of the play’s many instances of sharp social satire, Peachum comments on the irony of the relationship between criminals and the legal profession:
A Lawyer is an honest Employment, so is mine. Like me too he acts in a double Capacity, both against Rogues and for ’em; for ’tis but fitting that we should protect and encourage Cheats, since we live by them.
Looking through his Book of Accounts, in which he diligently records his finances, Peachum notes that he works in a “double Capacity.” Here, he references the two-fold nature of his occupation: first, he collects stolen items from criminals and sex workers, selling them for a profit. Second, he turns those same criminals in to the authorities, collecting the bounty on their heads. Cynically, he notes that he must "protect and encourage Cheats," since they are the source of his income, both through their criminal activity and through their bounties. Peachum, then, recognizes the situational irony of his scheme: he cooperates with the criminal justice system to punish thieves, but he's motivated only by profit and, in fact, must support those criminals until he turns them in.
Further, Peachum satirizes the legal profession, which he claims operates on a similar basis. Lawyers must prosecute criminals in court, fulfilling an essential role in the punishment of crime. And yet, at the same time, lawyers make their livelihoods through this work. Therefore, he claims that lawyers must in fact “encourage” the crime from which they profit. If there were no crime, he implies, then the lawyers would be out of work, as would Peachum himself. Peachum’s speech thus presents criminals and those who punish them as being two sides of the same coin, ultimately suggesting that they are in some way reliant upon one another.
When Filch claims that he is considering “going to sea” or working on a ship to avoid the dangers of the criminal lifestyle, Mrs. Peachum ironically commands him to learn the Bible so that he can commit crime without fear of repercussion:
Poor Lad! how little does he know as yet of the Old-Baily! For the first Fact I’ll insure thee from being hang’d; and going to Sea, Filch, will come time enough upon a Sentence of Transportation. But now, since you have nothing better to do, ev’n go to your Book, and learn your Catechism; for really a Man makes but an ill Figure in the Ordinary’s Paper, who cannot give a satisfactory Answer to his Questions.
Here, the selfish Mrs. Peachum behaves in a surprisingly maternal manner, treating her servant like an adopted son. She claims that she will protect him “from being hang’d,” and she dismisses his interest in “going to sea,” as he will likely be sentenced to work in an overseas penal colony anyway. Instructing Filch to stop wasting time with fantasies of working on a ship, she tells him to “go to your Book and learn your Catechism.”
Her command is an ironic inversion of conventional Christian morality. An 18th-century audience might expect that a young man would be instructed to read the Bible in order to better familiarize himself with ideas and morals central to the Christian faith. Regular readings from the Bible were considered essential to a Christian upbringing. Here, however, Mrs. Peachum has a very different motive: in the 18th century, clergymen could claim exemption from secular courts, choosing instead to be tried under the more lenient ecclesiastical courts administered by the church. Mrs. Peachum’s advice, then, is pragmatic rather than spiritual. If Filch learns to memorize some “Catechism” or summaries of passages of the Bible, then he can claim to be clergy and escape a death sentence.
