In the preceding sections reference has been made to unilateral contracts exclusively. In bilateral contracts a further element complicates the problem. In such contracts the consideration must be good on both sides. If, therefore, a promise is made by A to do something lawful while B promises to do two things, one lawful and one unlawful, there can be no recovery oh either side. B cannot recover because part of the consideration for the defendant A's promise is unlawful. A cannot recover even on B's lawful promise because since his own promise is unenforceable, it is itself insufficient consideration for B's promise.46a If, however, A performs his promise, the situation becomes the same as if the contract had originally been unilateral, and A can recover on B's lawful promise.47 If the legal portion of a bilateral contract is severable, legal promises on one side being wholly supported by legal promises on the other, and the illegal portion of the contract does not go to its essence, the legal part may be enforced.48
43 Towle v. Blake, 38 Me. 528; Goodwin v. Clark, 65 Me. 280; Robinson v. Green, 3 Mete. 159; Rundlett p. Weeber, 3 Gray, 263; Holt v. O'Brien, 15 Gray, 311; Chase v. Burkholder, 18 Pa. St. 48.
44 See Barrie v. Earle, 143 Mass. 1, 8 N. E. 639, 58 Am. Rep. 126.
45 Boyd v. Eaton, 44 Me. 51, 69
Am. Dee. 83; Walker v. Lovell, 28 N. H. 138, 61 Am. Dec. 605; Carleton v. Woods, 28 N. H. 290.
46 Wirth v. Roche, 92 Me. 383, 42 Atl. 794 (bottles in which beer was illegally sold); Biigh v. James, 6 Allen, 570 (casks in which liquor was illegally sold).