"In conspiracy cases a clear distinction is to be made between the existence of a conspiracy and the participation of each of the alleged conspirators in it. Conspiracy is the agreement of two or more persons to do an unlawful act or to do a lawful act by unlawful means and it is the fact of the agreement, or combination, to engage in a common enterprise which is the nub of the offence. This fact can seldom be proved by direct evidence of the making of an agreement and must in almost all cases be proved as a matter of inference from other facts, that is to say, by circumstantial evidence. For this purpose, evidence may be led which includes the acts or declarations of one alleged conspirator made outside the presence of the others provided such evidence is not led to prove against the others the truth of any assertion or implied assertion made by the actor or the maker of the statement. It may take the form of evidence of separate acts or utterances from which the fact of combination might be inferred. Led in that way, it is not hearsay and is not dependent upon some circumstance to take it outside the hearsay rule, such as an implied authority making the acts and words of one the acts and words of the other. Thus it was said in Tripodi[14] that proof of the crime of conspiracy 'may well consist in evidence of the separate acts of the individuals charged which, although separate acts, yet point to a common design and when considered in combination justify the conclusion that there must have been a combination such as that alleged in the indictment'. For example, it may be possible in a case of conspiracy to commit armed robbery to conclude from the fact that one accused wearing a disguise was present in a bank at the same time as another accused, similarly disguised, was waiting outside the bank in a motor vehicle with the motor running, that both were engaged in a common enterprise to rob the bank. For the purpose of reaching that conclusion it is permissible to use the acts and declarations of each, even in the absence of the other, not as proof of the truth of any assertion or implied assertion of the participation of the other, but as facts from which the combination might be inferred. Utterances for this purpose may be regarded as facts no less than acts and, indeed, in the United States are sometimes called verbal acts. In the example given it would be possible to reach the conclusion, admitting the evidence for the purpose described and considering it against each accused separately, not only that there was a conspiracy but also that each of the two accused was a participant.
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However, it is not in all cases that evidence of the separate acts of the alleged conspirators will prove both the fact of combination and their participation. Of course, if the evidence fails to prove a combination at all then that is an end of the matter. But if it proves a combination, although not the participation of an individual alleged to be a conspirator, then the question arises whether there are circumstances in which evidence of the acts and declarations of other participants, outside the presence of the individual, may be led against him, not as separate facts from which, when combined with other facts, an inference of combination may be drawn, but as evidence of his own participation."