How far and in what cases opinion is evidence, is next to be considered; in ordinary matters where, from a statement of facts, the jury, in the exercise of sound and ordinary understanding, are capable of arriving at a just conclusion, the opinion of a witness is neither requisite or admissible; but in matters of science it is otherwise, provided that he backs his opinion by such reason as may be satisfactory to the understanding of his hearers; and this is the principal qualification of a medical witness, that he make himself _intelligible to ordinary comprehensions_. No man is bound to give any evidence by which he may render himself liable to any criminal prosecution. At the Old Bailey Sessions, in June, 1821, Mr. _George Patmore_ was tried for the murder of _John Scott_, in a duel. Mr. _Pettigrew_, (a surgeon,) was the first witness called. Mr. Justice _Bailey_.—Mr. _Pettigrew_, I think it necessary to give you this caution, if you think the evidence, which you are about to give likely to expose you to a criminal prosecution, you are not bound to give it. Mr. _Pettigrew_. My Lord, I am not competent to form any opinion of my legal guilt; I have not taken the part of principal or second. The part which I have taken was merely to exercise my professional duty; in that I do not think there is any moral guilt. Mr. Justice _Bailey_. If you went (knowing a duel was to take place) for the purpose of giving surgical assistance, I apprehend that you are liable to a criminal prosecution. Mr. _Pettigrew_. Then, my Lord, I must decline answering any questions. Mr. Justice _Bailey_. I recollect having seen a surgeon of eminence tried in this court, on a similar occasion. Neither Mr. _Pettigrew_, nor his assistant, were examined. Dr. _Darling_, who had attended the deceased after he had received his wound, deposed that he heard Mr. _Scott_ on his death bed say—— Mr. Justice _Bailey_. Did Mr. _Scott_ at that time think himself in danger: did he give up all hopes of recovery? Dr. _Darling_. No. To the last he entertained hopes of recovery. Mr. Justice _Bailey_. The declaration made by a dying man cannot be received as evidence, unless the party at the time of making it were satisfied that recovery was impossible. We have before noticed the limitation with which we believe this supposed rule must be taken.