Bill hired Seth to drive his racing car in a race. Luke, a friend of Seth, promised to pay Seth $3,000 if he won the race. Seth won the race, but Luke refused to pay the $3,000. Luke contended that no legally binding contract had been formed because he had received no consideration from Seth for his promise to pay the $3,000. Seth sued Luke for breach of contract, arguing that winning the race was the consideration given in exchange for Luke's promise to pay the $3,000. What rule of law discussed in this chapter supports Luke's claim?