Although of the utmost consideration, the extent of this enquiry would seem to be very limited, and contained in this simple question. Is the person accused, of insane mind? If he be insane he will be acquitted on the _proof_ of his insanity—if he be not of insane mind, he must be treated as an ordinary delinquent. In those cases where the prisoner is so bereft of his reason, that any twelve men would not entertain a different opinion, where numerous evidences appear to testify to repeated acts of insanity, which are so manifest that they cannot be otherwise interpreted; and where he has been confined and treated for this malady, the physician will have an easy duty to perform: but it is in cases which appear to be involved in difficulty, where the disorder, although existing and directing the actions, is not so ostensibly developed that the medical evidence becomes important, and capable by sagacity, experience and truth, of explaining and characterizing the state of the person’s intellect. The important duty which the medical practitioner has to perform, when he delivers his testimony before a court of justice, should be clearly defined, conscientiously felt, and thoroughly understood,—his opinion ought to be conveyed in a perspicuous manner; he should be solemnly impressed that he speaks upon oath, the most sacred pledge before God between man and man—and that the life of a human being depends on the clearness and truth of his deposition. He is not to palm on the court the trash of medical hypothesis as the apology for crime; neither should the lunatic receive his cure at the gallows by the infirmity of his evidence—but above all his opinion should be so thoroughly understood by himself; so founded in experience and fortified by reason, that it may resist the blandishments of eloquence and the subtil underminings of cross-examination. The physician should not come into court merely to give his opinion—he should be prepared to explain it, and able to afford the reasons which influenced his decision:—without such elucidation, Opinion becomes a bare Dictum, and endeavours to claim precedence, without courtesy or obligation to science.