When are the opinions of experts relevant? What is their evidentiary value? What are the differences between an expert and an ordinary witness? Discuss fully and illustrate your answer.

The General Rule and Its Exception

The law of evidence operates on a foundational principle: witnesses must speak to facts, not to opinions. As the Supreme Court stated in Mobarik Ali Ahmed v State of Bombay (AIR 1957 SC 857), permitting a witness to express opinions on matters before the court would amount to a delegation of judicial function — and the drawing of inferences from facts is the exclusive province of the judge. Opinion, therefore, is ordinarily irrelevant.

But the law is not rigid where human knowledge is limited. A judge presiding over a murder trial involving cyanide poisoning, or a civil dispute about the structural failure of a harbour wall, cannot be expected to possess the knowledge of a toxicologist or a marine engineer. The law therefore carves out a carefully delimited exception — the opinion of a person who is specially skilled becomes relevant when the court itself cannot form an informed judgment on a technical matter without that assistance.

The Statutory Foundation: Section 45 IEA and Section 39 BSA

Section 45 of the Indian Evidence Act, 1872 provides that when the Court has to form an opinion upon a point of foreign law, science, art, or as to the identity of handwriting or finger impressions, the opinions of persons specially skilled in those subjects are relevant facts. Such persons are called experts.

The Bharatiya Sakshya Adhiniyam, 2023, consolidates and modernises this position through Section 39, which merges the old Sections 45 and 45A into a single provision. Critically, Section 39 BSA adds an important expansion — opinions are now relevant on "any other field" beyond the enumerated categories of science, art, foreign law, handwriting, and fingerprints. This is a significant legislative recognition of the explosion of specialised knowledge in the modern world. Section 39(2) BSA further makes the opinion of the Examiner of Electronic Evidence (designated under Section 79A of the Information Technology Act, 2000) a relevant fact in proceedings involving information stored or transmitted in any computer resource or electronic or digital form — a provision particularly vital in an age of cybercrime and digital contracts.

The three illustrations to Section 45 (and Section 39 BSA) are instructive:

• Illustration (a): If the question is whether A died of poison, the opinions of experts regarding the symptoms produced by that poison are relevant. Here the expert need not have personally examined the deceased — he may opine on the basis of the symptoms communicated to him, because his expertise lies in the pharmacological relationship between the poison and bodily reactions.

• Illustration (b): If A's sanity at the time of an act is in question, experts may opine whether the symptoms exhibited by A indicate unsoundness of mind, and whether such unsoundness typically renders a person incapable of understanding the nature or wrongfulness of the act.

• Illustration (c): Where the authorship of a document is disputed, experts may compare the document in question with a proven specimen of the alleged writer's handwriting and express an opinion.

Who is an Expert?

The Act does not prescribe any particular qualification or attainment as a condition for being recognised as an expert. The test is a functional one: Is he peritus? Is he skilled? Does he possess adequate knowledge? As the Supreme Court put it in State of HP v Jai Lal (AIR 1999 SC 3318), "an expert witness is one who has made the subject upon which he speaks a matter of particular study, practice, or observation, and he must have a special knowledge of the subject."

The words "science or art" in Section 45 are given a deliberately wide meaning. As Field's classical commentary observed — and as Indian courts have repeatedly affirmed — "art" is not confined to fine arts but includes handicraft, trade, and profession; "science" is not limited to the natural sciences but encompasses any field of systematised knowledge. So a goldsmith who can identify gold by rubbing it on a touchstone is an expert in the relevant sense (Assistant Collector of Central Excise v VP Syed Mohd, AIR 1983 SC 168). An excise inspector with twenty-one years of service in testing liquor is an expert for that purpose (SC Batra v State of UP, AIR 1974 SC 639). A police officer trained in handling firearms is competent to testify whether a gun is in working condition (Jarnail Singh v State of Punjab, AIR 1999 SC 321). Academic qualifications, though often relevant, are not the exclusive gateway.

Equally important is the converse: a caddie who witnesses a golf player kill another caddie with a stroke is not an expert on whether the act was rash or negligent — that falls within ordinary lay assessment. Similarly, in MS Reddy v State Inspector of Police, ACB, Nellore (1993 Cr LJ 558, AP), an engineer who had acquired departmental knowledge in the course of service was held not to be an expert in the relevant sense.

Facts Supporting or Inconsistent with Expert Opinion: Section 46 IEA / Section 40 BSA

Section 46 IEA (Section 40 BSA) performs a critical evidentiary function. It provides that facts not otherwise relevant become relevant if they support or are inconsistent with the opinion of an expert, when that opinion itself is relevant. The two illustrations are particularly revealing:

• Illustration (a): If the question is whether A was poisoned by a certain substance, the fact that other persons poisoned by that substance exhibited the same symptoms that experts attribute to it is relevant — whether the experts affirm or deny the connection.

• Illustration (b): If the dispute is whether a harbour's obstruction was caused by a sea-wall, the fact that other harbours similarly situated but without any sea-wall began to be similarly obstructed at about the same time becomes relevant — because it tends to rebut the causal link.

The classical English case of Folkes v Chadd (1782, 3 Doug. 157) is illustrative: engineers disagreed about whether a sea-wall caused the silting of a harbour, and the court permitted evidence of the condition of other harbours to elucidate the expert opinion. This is the very principle codified in Section 46. The provision is essentially a recognition that expert opinion does not stand in isolation — it must be tested against facts in the real world.

Grounds of Opinion: Section 51 IEA / Section 45 BSA

Section 51 IEA (Section 45 BSA) provides that whenever the opinion of any living person is relevant, the grounds on which that opinion is based are also relevant. The illustration to this section is precise: an expert may give an account of the experiments performed by him for the purpose of forming his opinion.

This provision is not merely procedural — it is foundational to the reliability of expert evidence. The Calcutta High Court, in a passage repeatedly cited by superior courts, put it well: "The duty of an expert is to depose and not to decide. The only function of the expert is to furnish the data with necessary scientific criteria so as to enable the judge to come to an independent conclusion." An opinion without reasons is an empty opinion. As the old maxim runs — opinion is no evidence without assigning the reason of such opinion.

The Evidentiary Value of Expert Opinion

Expert opinion occupies a peculiar place in the hierarchy of evidence — it is relevant but not conclusive. The court is neither bound by it nor free to ignore it arbitrarily. Several settled principles govern its weight:

First, expert opinion is essentially of an advisory character. In Ramesh Chandra Agrawal v Regency Hospital Ltd (AIR 2010 SC 806), the Supreme Court reaffirmed that a medical witness called as an expert is not a witness of fact, and his evidence is advisory in nature. Once the court accepts the expert's reasoning, it becomes the court's own opinion — not the expert's.

Second, expert opinion ordinarily requires corroboration before it can be the sole basis of a finding, particularly in handwriting identification. The court stated in Murari Lal v State of MP (AIR 1980 SC 531) that before acting on handwriting expert evidence, it is usual to see if it is corroborated by other clear or circumstantial evidence. The sole evidence of a handwriting expert is generally not sufficient for a definitive finding.

Third, where ocular evidence and medical expert evidence conflict, the former ordinarily prevails — unless the medical evidence completely rules out the version of the eyewitness. The Supreme Court in Yogesh Singh v Mahabeer Singh (2017 Cr LJ 291) stated the principle succinctly: medical evidence is only corroborative and cannot displace positive direct evidence unless it renders the direct evidence physically impossible.

Fourth, the court retains its independent judgment. As held in Law Society of India v Fertilizers and Chemicals Travancore Ltd (AIR 1994 Ker 308), experts cannot act as judges or jury — the court must assess their opinion and decide finally. The court does not become functus officio (discharged of its duty) merely because an expert has spoken. Expert opinion is not a substitute for judicial reasoning; it is an aid to it.

Fifth, the independence and impartiality of the expert materially affects the weight to be given to his opinion. Where an expert is employed by the very party calling him and has a direct financial stake in the outcome, his evidence is to be received with heightened caution. In Novapan India Ltd v CCEC (1994, 72 ELT 769), the tribunal was justified in rejecting the opinion of an expert who was not an independent person. Government scientific experts, on the other hand, stand on a somewhat different and more reliable footing, as the Supreme Court noted in Prem Sagar Manocha v State NCT of Delhi (2016 4 SCC 571 — the Jessica Lal ballistics case).

Expert Witness vs. Ordinary Witness: A Fundamental Distinction

The distinction between an expert and an ordinary witness is not merely academic — it has profound consequences for the nature, scope, and reliability of their testimony.

Lord Bingham CJ put the distinction between the two elegantly in R v Steven Jones (1997 1 Cr App R 86): "Expert witnesses, although inevitably varying in standing and experience, are interchangeable in a way in which factual witnesses are not." The ordinary witness is unique — tied to a specific perception of a specific event at a specific time. The expert is fungible in the sense that any suitably qualified person in the field may review the same material and offer a professional opinion.

However, an expert must never assume the role of an advocate. He does not belong to the party that hired him — his primary duty is to the court. Where an expert surrenders his objectivity and becomes a paid instrument of the party calling him, his evidence loses credibility, as Lord Jessel MR caustically noted in Abinger v Ashton (1873, 17 LR Eq 358): experts often consider themselves "paid agents" rather than neutral witnesses, and their minds are naturally biassed toward those who remunerate them.

A Practical Illustration

Consider a prosecution for murder by poisoning. The body of the deceased is exhumed, and a forensic pathologist performs an autopsy. He gives the following testimony:

1. He found no oedema (water-logging) of the lungs — this is a fact he personally observed.

2. Absence of oedema in cases of drowning typically indicates that the person was dead before entering the water — this is a scientific generalisation, i.e., a fact within medical science.

3. In the circumstances of this case, based on all the signs found, his opinion is that the death was homicidal, not suicidal — this is his expert opinion.

The court may accept items (1) and (2) as established. Item (3) — the opinion — it will weigh against all other evidence, including the eyewitness account, the circumstances of the recovery of the body, and any forensic chemistry reports. If the eyewitness account is reliable and consistent with the pathologist's findings in items (1) and (2), the court may act on it. But the court is the final arbiter — it neither surrenders to the expert's conclusion nor dismisses it without reason.

The BSA's Modernisation

The Bharatiya Sakshya Adhiniyam, 2023 takes Section 45 IEA forward in two meaningful ways. By merging Sections 45 and 45A, and by adding "any other field" as a ground for expert opinion under Section 39(1), the BSA formally acknowledges that the frontiers of human knowledge cannot be exhaustively listed in a statute. Cybersecurity analysis, digital forensics, artificial intelligence, environmental science — all of these can now, without statutory ambiguity, attract expert opinion evidence. Section 39(2) specifically anchors the Examiner of Electronic Evidence as an expert for matters involving information in computer resources, acknowledging that a trial judge examining metadata or encrypted communication is as helpless without an expert as a judge examining an autopsy report without a pathologist.

The law of expert evidence thus reflects a deep constitutional truth about the judicial process — that the judge is the master of law, but not necessarily the master of every science. The expert enters the courtroom not to usurp the judge's function, but to illuminate facts that would otherwise remain beyond the court's competent understanding. The moment the expert forgets this boundary and begins to advocate rather than advise, his opinion loses not just credibility, but its very claim to be received as expert evidence.