In Greenleaf it is laid down:—
"That courts-martial are bound, in general, to observe the rules of the law of evidence by which the courts of criminal jurisdiction are governed." (3 Greenleaf, section 467.)
This covers all the great general principles of evidence, the points of difference being wholly as to minor matters. And it is also affirmed in Benet:—
"That it has been laid down as an indisputable principle, that whenever a legislative act erects a new jurisdiction, without prescribing any particular rules of evidence to it, the common law will supply its own rules, from which it will not allow such newly-erected court to depart. The rules of evidence, then, that obtain in the criminal courts of the country must be the guides for the courts-martial; the end sought for being the truth, these rules laid down for the attainment of that end must be intrinsically the same in both cases. These rules constitute the law of evidence, and involve the quality, admissibility, and effect of evidence and its application to the purposes of truth." (Benet, pp. 226, 327.)
Therefore, all the facts that tend against the accused, and all those that mate for her, are to be weighed and are to operate upon her conviction or acquittal precisely as they would in a court of law. If they present a case such as would there convict her she may be found guilty here; and if, on the other hand, the rules of law upon these facts would raise any presumption or create any doubt, or force any conclusions that would acquit her in a court of law, then she must be discharged, upon the same principles by the commission. This is a point which, in our judgment, we cannot too strongly impress upon the minds of her judges. The extraordinary character of the crime—the assassination that removed from us the President of the United States—makes it most desirable that the findings of this tribunal shall be so well founded in reason as to satisfy and secure public confidence, and approval; for many of the most material objects of the prosecution, and some of the most important ends of justice, will be defeated and frustrated if convictions and acquittals, and more especially the former, shall be adjudged upon the grounds that are notoriously insufficient.
Such a course of action would have a tendency to draw sympathy and support to the parties thus adjudged guilty, and would rob the result of this investigation of the wholesome support of professional and public opinion. The jurisdiction of the commission, for example, is a matter that has already provoked considerable criticism and much warm disapproval; but in the case of persons clearly found to be guilty, the public mind would easily overlook any doubts that might exist as to the regularity of the court in the just sentence that would overtake acknowledged criminals. Thus, if Booth himself and a party of men clearly proved, by ocular evidence or confession, to have aided him, were here tried and condemned, and, as a consequence, executed, not much stress, we think, would be laid by many upon the irregularity of the mode by which they should reach that just death which all good citizens would affirm to be their deserts. But the case is far different when it affects persons who are only suspected, or against whom the evidence is weak and imperfect; for, if citizens may be arraigned and convicted for so grievous an offense as this upon insufficient evidence, every one will feel his own personal safety involved, and the tendency would be to intensify public feelings against the whole process of the trial. It would be felt and argued that they had been condemned upon evidence that would not have convicted them in a civil court, and that they had been deprived, therefore, of the advantage, which they would have had for their defense. Reproach and contumely upon the government would be the natural result, and the first occasion would arise in all history for such demonstrations as would be sure to follow the condemnation of mere citizens, and particularly of a woman, upon evidence on which an acquittal would follow in a civil court. It is, therefore, not only a matter of the highest concern to the accused themselves, as a question of personal and private right, but also of great importance upon considerations of general public utility and policy, that the results of this trial, as affecting each of the accused, among them Mrs. Surratt, shall be rigidly held within the bounds and limitations that would control in the premises, if the parties were on trial in a civil court upon an indictment equivalent to the charges and specifications here. Conceding, as we have said, the jurisdiction for the purpose of this branch of the argument, we hold to the principle first enunciated as the one great, all-important, and controlling rule that is to guide the commission in the findings they are now about to make. In order to apply this principle to the case of our client, we do not propose to range through the general rules of evidence with a view to seeing how they square with the facts as proven against her. In the examination of the evidence in detail, many of these must from necessity be briefly alluded to; but there is only one of them to which we propose in this place to advert specifically, and that is the principle that may be justly said to lie at the foundation of all the criminal law—a principle so just, that it seems to have sprung from the brain of Wisdom herself, and so undoubted and universal as to stand upon the recognition of all the times and all the mighty intellects through and by which the common law has been built up. We allude, of course, to that principle which declares that "every man is held to be innocent until he shall be proven guilty"—a principle so natural that it has fastened itself upon the common reason of mankind, and been immemorially adopted as a cardinal doctrine in all courts of justice worthy of the name. It is by reason of this great underlying legal tenet that we are in possession of the rule of law, administered by all the courts, which, in mere technical expression, may be termed "the presumption of innocence in favor of the accused." And it is from hence that we derive that further application of the general principle, which has also become a rule of law, and of universal application wherever the common law is respected (and with which we have more particularly to deal), by which it is affirmed, in common language, that in any prosecution for crime "the accused must be acquitted where there is a reasonable doubt of his guilt." We hardly think it necessary to adduce authorities for this position before any tribunal. In a civil court we certainly should waive the citations, for the principle as stated would be assumed by any civil judge and would, indeed, be the starting point for any investigation whatever. Though a maxim so common and conceded, it is fortified by the authority of all the great lights of the law. Before reference is made to them, however, we wish to impress upon the minds of the court another and important rule to which we shall have occasion to refer:—
"The evidence in support of a conspiracy is generally circumstantial" (Russell on Crimes, Vol. ii., 698.)
In regard to circumstantial evidence, all the best and ablest writers, ancient and modern, agree in treating it as wholly inferior in cogency, force, and effect, to direct evidence. And now for the rule that must guide the jury in all cases of reasonable doubt:—
"If evidence leave reasonable ground for doubt, the conclusion cannot be morally certain, however great may be the preponderance of probability in its favor." (Wills on Circumstantial Evidence. Law Library, Vol. xli.)
"The burden of proof in every criminal case is on the government to prove all the material allegations in the indictment; and if, on the whole evidence, the jury have a reasonable doubt whether the defendant is guilty of the crime charged, they are bound to acquit him. If the evidence lead to a reasonable doubt, that doubt will avail in favor of the prisoner." (1 Greenleaf, section 34—Note.)