“An expert is one who has made a subject upon which he speaks or renders his opinion, a matter of particular study, practice or observation and has a special knowledge thereof. His knowledge must be within the recognized field of expertise and he essentially has to be qualified in that discipline of study. It has been propounded that an expert is not a witness of fact and its evidence is really of an advisory character and it is his duty to furnish to the Judge/Court the necessary scientific criteria for testing the accuracy of the conclusions so as to enable the Judge/Court to form his/its independent Judgment by the application of such criteria to the facts proved by the evidence. Referring to Section 45 of The Evidence Act, 1892 which makes the opinion of an expert admissible, it has been underlined that not only an expert must possess necessary special skill and experience in his discipline, his opinion must be backed by reason and has to be examined and cross-examined to ascertain the probative worth thereof. That it would be unsafe to convict the person charged on the basis of expert opinion without any independent corroboration has also been indicated. It has been held that the evidentiary value of the opinion of an expert depends on the facts upon which it is based and also the validity of the process by which the conclusion has been reached. The decisions underline that the Court is not to subjugate its own judgment to that of the expert or delegate its authority to a third party but ought to assess the evidence of the expert like any other evidence.”
– Hon’ble Justice Pinaki Chandra Ghose, State of Karanataka v. Selvi J. Jayalalitha, [Criminal Appeal Nos. 300-303 of 2017].