The Common-Law Rule

The Common-Law Rule

At common law, a person charged with conspiracy cannot defend by proving that facts outside the conspirators' knowledge made their ultimate objective impossible to achieve. The agreement and the intent to commit the unlawful act establish the offense, and learning later that the plan could never have worked leaves both of those ingredients exactly where they were.¹ The rule is short, but it rests on a point about what the prosecution has to prove, and that point is worth working through before testing the rule on facts.

The defendants in these cases are charged with conspiracy, and the object crime they hoped to commit is not what is on trial. Conspiracy is complete when the parties agree, with the intent to achieve an unlawful objective, or, in jurisdictions that demand an overt act, when one member takes a step in furtherance of the agreement.² Success of the objective appears nowhere in that definition. An impossibility argument therefore asks the court to excuse the defendants for lacking something the prosecution was never required to prove. Attempt law asks how close the defendant's conduct came to the finished crime, so a plan that could never be finished can matter there; in State v. Moretti (1968) the court explained that conspiracy differs because the charge focuses primarily on the defendants' intent while an attempt case centers on their conduct, which makes the impossibility of consummating the contemplated crime less pertinent.³

The example changes only one thing from a plan that succeeds: a fact about the world that neither defendant knew. Their agreement, their purpose, and their readiness to act are the same as if the woman had been alive. Students lose points here when they ask whether the defendants could have committed rape, because the charged offense is conspiracy. Whether the same defendants could be convicted of attempted rape is a separate question that runs on attempt law and can come out differently, as the next illustration shows.

A familiar illustration makes the split visible. A and B agree to kill C while C sleeps, and A buys a gun to further the plan. Unknown to them, C dies of a heart attack several hours before B shoots at what appears to be a sleeping man. In some jurisdictions B's shot is treated as legal impossibility that defeats a charge of attempted murder, and in others it is treated as factual impossibility that leaves B guilty of attempt, yet A and B are guilty of conspiracy to murder C either way.⁴ The agreement and the step taken in furtherance of it were complete before C died, so nothing that happened afterward could undo the offense. The same illustration adds that the result would hold even if C had died right after the agreement and the pair had done nothing more once they learned of it.

Why should the law treat a plan that failed over a hidden fact like a plan that succeeded? Whether an objective can be reached usually depends on facts the conspirators do not control and typically do not know, such as whether the safe has been emptied or whether the target is still alive. A rule that excused them whenever those facts happened to run against the plan would make guilt depend on luck instead of on what the defendants agreed to do and meant to do. Conspirators whose plan collapses over a hidden fact are no less blameworthy than those whose plan works, and the next section develops the further point that criminal combinations are dangerous apart from the danger of attaining the particular objective.⁵

The rule covers one kind of failed plan, and the limit matters. It disposes of the claim that hidden facts made the objective unreachable, which attempt law calls factual impossibility. It does not decide a case in which the plan fails because the conduct the parties agreed on is not criminal in the first place.

This second kind of failure is legal impossibility, which the section on classifying the mistake takes up, and it is why the rule here is stated as a rule about factual impossibility only. Two further limits come after that. A conspiracy charge can still fail where a defendant lacked the intent the target crime requires, or where the supposed agreement was with someone only pretending to join. The next section first explains why the common law reached the rule, and the later sections mark where it stops.