Ian Ayres, teaching American Contract Law at Yale (from his Coursera course), examines illegality as a defense against enforcing a promise. The case study is Wallis v. Smith, in which a man sued a woman for breach of an oral agreement after she became pregnant despite representing that she was using contraception, the so called unwanted child case. Ayres walks through why courts refuse to enforce certain bargains even when the parties actually agreed to them, framing illegality alongside other formation defenses covered in the course. He uses the case to probe where courts draw the line between private ordering and public policy, and what kinds of promises society decides are simply not for sale. The segment runs about twelve minutes and functions as one unit within a longer sequence on defenses to contract formation.