The two elements of mental state required by conspiracy are the intent to agree and the intent to promote the unlawful objective of the conspiracy. The first of these elements is almost indistinguishable from the act of agreement. Agreement is in any case morally neutral; its moral character depends upon the nature of the objective of agreement. It is the intention to promote a crime that lends conspiracy its criminal cast.
Some crimes do not require an intention to cause the prohibited result. Manslaughter, for example, may be committed by a person who kills another by his act of driving carelessly. These crimes may not be the basis of a conspiracy, however, since two people could not be said to agree together to kill another carelessly. The nature of the requirement of agreement, therefore, limits the objectives of conspiracy to those crimes that are committed by intentional actions.
Problems arise, however, in determining the sense of intention that is required. Does it include acting with knowledge of the probable results of one's action, or is it confined to acting with a purpose to attain such results? The question has most frequently arisen in the case of suppliers who furnish goods to members of a conspiracy with knowledge of their intended illegal use. Examples include the supplying of yeast and sugar to a group known to be using them to engage in illegal production of whiskey (United States v. Falcone, 311 U.S. 205 (1940)), or the furnishing of medical drugs by a manufacturer that knows they will be used for nonmedical and illegal purposes (Direct Sales Co. v. United States, 319 U.S. 703 (1943)).
Some courts have found it enough to convict the supplier for an illegal conspiracy with the user when the supplier knew of the illegal use. The justification for this position is that the supplier has knowingly furthered a crime and has no interest in doing so that is worthy of protection (MPC, 1960, commentary on § 5.03). However, the majority view is to the contrary: the supplier must be shown to have had a purpose to further the illegal objectives of the user (MPC, 1962, § 5.03(1)). In the language of Judge Learned Hand, "he must in some sense promote their venture himself, make it his own, have a stake in its outcome" (United States v. Falcone, 109 F.2d 579, 581 (1949)). This might be demonstrated by evidence of the sale of unusually large quantities of goods, particularly where such goods are legally restricted; by evidence of inflated charges or of the sale of goods with no legitimate use; or by evidence that sales to an illegal operation have become a dominant proportion of the seller's business.
The reasons for requiring a stake in the venture are twofold. First, the act of agreement necessarily imports a purpose; indifference to illegal use by another of what one supplies him for otherwise legitimate reasons does not constitute an agreement. Second, making the supplier liable in these situations whenever a jury decides that he knew of the illegal use imposes an undue burden on legitimate business since to avoid liability suppliers would be obliged to police the intended uses of their purchasers. By taking into account the social usefulness of the commercial activity and the magnitude of the seller's contribution to the crime, the majority rule strikes a balance between the needs of business enterprises to operate without oppressive restriction and of society to protect itself against crime.
Sometimes the issue arises whether a mistake of fact that would not defeat liability for the object offense nevertheless defeats liability for conspiracy. The argument that it does is sometimes couched in logical or conceptual terms: "While one may, for instance, be guilty of running past a traffic light of whose existence one is ignorant," Judge Learned Hand wrote in another famous decision, "one cannot be guilty of conspiring to run past such a light unless one supposes that there is a light to run past" (United States v. Cummins, 123 F.2d 271, 273 (1941)). But other courts, including the U.S. Supreme Court in United States v. Feola, 420 U.S. 672, 693 (1975), have taken a more pragmatic stance, reasoning that the mental state element of the conspiracy charge should mirror that of the substantive offense "unless one of the policies behind the imposition of conspiratorial liability [would] not [be] served by such a result." The Model Penal Code makes this an issue to be resolved on a case-by-case basis (MPC 1985, commentary on § 5.03, at 4.13).