The appellants, Frank D'Angelo, Richard Delcazal and Wade A. Freman, were tried together and convicted on different offenses arising from a scheme to distribute marijuana.1 We affirm the convictions.

Appellants Delcazal and Freman raise two issues on appeal. First, they argue that the district court erred in allowing the government's chief witness a co-conspirator not indicted by the grand jury to testify concerning a marijuana delivery that was Allegedly intended originally for Delcazal but was diverted to another buyer. No objection was made to the testimony at trial and there was no error in its admission. The testimony was evidence of overt acts committed by other conspirators during the life of the conspiracy and as such was admissible. United States v. Elliot, 571 F.2d 880, 911 (5th Cir. 1978); United States v. Perez, 489 F.2d 51, 70 (1973), Cert. denied, 417 U.S. 945, 94 S. Ct. 3067, 41 L. Ed. 2d 664 (1974).

Second, Decazal and Freman claim error in the district court's refusal to grant a motion for an indefinite continuance to obtain the presence of a witness the defendants had first tried to locate only three days prior to trial. The court denied the motion on the grounds that the defendants had failed to exercise due diligence in attempting to procure the witness. The exercise of due diligence is a prerequisite to the grant of such motions. United States v. Uptain, 531 F.2d 1281, 1286 (5th Cir. 1976). A district court's ruling on such a motion will not be disturbed unless an abuse of discretion is shown. United States v. Smith, 591 F.2d 1105, 1110 (5th Cir. 1979). No abuse of discretion occurred here.

The sole issue raised on appeal by the appellant D'Angelo concerns a note sent to the trial judge by the jury after the jury reached its verdict. The note reads:

We have had great difficulty deciding and coming to agreement about Frank D'Angelo and have decided as we did only because we believe He knew about the transactions and thus was a conspirator. (Emphasis by the jury)

After discussing the significance of the note with counsel out of the jury's presence, the court decided that the note could not impeach the jury's general verdict of guilty. The jury was brought into the courtroom and polled, and all jurors adhered to the verdict. None of the jurors made any reference to the note or in any other way attempted to qualify or elaborate on the verdict.2

We cannot divine with certainty why the jury sent the note or what it was intended to mean. It is entirely possible that the note means what D'Angelo claims it means: that the jury found D'Angelo guilty only because he Knew about the transactions and thus was a conspirator. Willful participation is an essential element of the crime of conspiracy; mere knowledge of a conspiracy does not itself make a person a conspirator. E. g., United States v. Binetti, 552 F.2d 1141 (5th Cir. 1977). If the jury convicted D'Angelo on the basis of knowledge alone, it misapplied the law.

The possibility that the jury misunderstood or even intentionally misapplied the law, however, does not warrant reversal of the conviction. D'Angelo's attack on the verdict is an attempt to expose the jury's collective mental process to judicial scrutiny. There is no claim that the court erred in giving the law to the jury as it did. In fact the court's instructions placed heavy emphasis on the necessity of a finding of willful participation in the conspiracy. There is no claim that evidence of knowing participation did not exist. Indeed, there could not be such a claim for there was testimony by a government witness of direct dealings with D'Angelo. There is no claim that the jury was corrupted by some extraneous influence, that the verdict was not unanimous, or that the verdict was anything other than a general verdict of guilty.3 Rather, D'Angelo's attack on the verdict is, as the trial judge characterized it, an attempt to use the jury's note to probe its process of deliberation and find out how and why the jury reached its verdict. That is the one form of attack on a verdict that has always been forbidden in Anglo-American criminal law.

A jury has an obligation to follow the law as it is given by the trial court, but it is a peculiar facet of the jury institution that once a verdict is rendered, no judicial inquiry is permitted into the jury's deliberative process to determine if in fact the court's instructions were properly followed. As the Supreme Court noted long ago, in that limited sense a jury passes both on law and fact, for " 'in practice, the verdict of the jury, both upon the law and the fact, is conclusive; because, from the nature of the proceeding, there is no judicial power by which the conclusion of law thus brought upon the record by that verdict can be reversed, set aside, or inquired into.' " Sparf v. United States, 156 U.S. 51, 80, 15 S. Ct. 273, 285, 39 L. Ed. 343 (1895) (quoting from Com. v. Anthes, 5 Gray 185).

Severe limitations exist on all attempts "to breach the shroud surrounding jury deliberations," United States v. Gipson, 553 F.2d 453, 457 (5th Cir. 1977). The rule of the common law is that a juror may not impeach his verdict. E. g., McDonald v. Pless, 238 U.S. 264, 35 S. Ct. 783, 59 L. Ed. 1300 (1915); United States v. Blackburn, 446 F.2d 1089, 1091 (5th Cir. 1971); Dickinson v. United States, 421 F.2d 630, 632 (5th Cir. 1970). The sanctity of the verdict may be impugned only by evidence of improper external influences on the jury's decision, E. g., Mattox v. United States, 146 U.S. 140, 13 S. Ct. 50, 36 L. Ed. 917 (1892); Stimack v. Texas, 548 F.2d 588, 589 (5th Cir. 1977); United States v. Howard, 506 F.2d 865, 868 (5th Cir. 1975), or by a post-verdict determination that the evidence did not warrant submission to the jury in the first instance. Inquiry into the conscience or compliance of jurors, by contrast, requires inquiry into a matter that essentially inheres in the verdict, Mattox v. United States, supra, 146 U.S. at 149, 13 S. Ct. at 53, and is thus strictly forbidden. See United States v. Howard, supra, 506 F.2d at 868 n. 3; Miller v. United States, 403 F.2d 77, 83 n. 11 (2nd Cir. 1968). With the exception for evidence concerning extraneous influence, the Federal Rules of Evidence bar juror testimony "as to any matter or statement occurring during the course of the jury's deliberation or to the effect of anything upon his or any other juror's mind or emotions as influencing him to assent or dissent from the verdict or indictment or concerning his mental processes in connection therewith." Fed.R.Evid. 606(b).

The necessary consequence of the rule against examination of the jury's mental process is that convictions must stand despite the presence of plausible suspicion that the jury's mental process was ill-conceived. In Davis v. United States, 47 F.2d 1071 (5th Cir. 1931), for example, a conviction was upheld despite the post-trial testimony of two jurors that the jury had considered the defendant's failure to take the stand in his own defense as evidence of guilt, and that the jurors had not heard the trial court's instruction that the jury was not to consider or discuss the defendant's failure to testify. Relying on McDonald v. Pless, supra, the court held that the improper argument of jurors within the jury room was not grounds for upsetting a verdict. 47 F.2d at 1072.

"(T)he essential feature of a jury lies in the interposition between the accused and his accuser of the commonsense judgment of a group of laymen." Williams v. Florida, 399 U.S. 78, 100, 90 S. Ct. 1893, 1906, 26 L. Ed. 2d 446 (1970). To preserve that essential feature, the law trusts that a jury will understand and follow the law as instructed, and it indulges the jury when apparent gaps in understanding or logic later surface.4 A jury, for example, may render logically inconsistent verdicts on different counts of an indictment or as to different co-defendants. It is not the duty of the court "to unravel the ratiocinations of the jury's collective logic." Odom v. United States, 377 F.2d 853, 857 (5th Cir. 1967). Nor may a court speculate that a verdict may have been the result of compromise, mistake or even carelessness. United States v. Dotterweich, 320 U.S. 277, 279, 64 S. Ct. 134, 135, 88 L. Ed. 48 (1943); Dunn v. United States, 284 U.S. 390, 393-94, 52 S. Ct. 189, 190-91, 76 L. Ed. 356 (1932). "Juries may indulge in precisely such motives or vagaries." United States v. Dotterweich, supra, 320 U.S. at 279, 64 S. Ct. at 135. If courts were permitted to retry such verdicts, the result would be that every jury verdict would either become the court's verdict or would be permitted to stand only by the court's leave. This would destroy the effectiveness of the jury process which substantial justice demands and the constitution guarantees.

AFFIRMED.