Costello hired Sagan to drive his racing car in a race. Sagan’s friend Gideon promised to pay Sagan $3000 if she won the race. Sagan won the race, but Gideon refused to pay the $3000. Gideon contended that no legally binding contract had been formed because he had received no consideration from Sagan in exchange for his promise to pay the $3000. Sagan sued Gideon for breach of contract, arguing that winning the race was the consideration given in exchange for Gideon’s promise to pay the $3000. What rule of law that we have discussed supports Gideon’s argument?