An expert opinion under Section 39 is only useful to a court if the surrounding facts can be tested against it. Section 40 of the Bharatiya Sakshya Adhiniyam, 2023 (BSA) supplies that companion rule. Where an expert opinion is relevant, facts that support or contradict it become relevant too — even if they would not, standing alone, satisfy the ordinary tests of relevance. It is a targeted grant of relevance, aimed squarely at making expert opinion meaningful in the courtroom.
40. Facts bearing upon opinions of experts. Facts, not otherwise relevant, are relevant if they support or are inconsistent with the opinions of experts, when such opinions are relevant.
Illustrations.
(a) The question is, whether A was poisoned by a certain poison. The fact that other persons, who were poisoned by that poison, exhibited certain symptoms which experts affirm or deny to be the symptoms of that poison, is relevant.
(b) The question is, whether an obstruction to a harbour is caused by a certain sea-wall. The fact that other harbours similarly situated in other respects, but where there were no such sea-walls, began to be obstructed at about the same time, is relevant.
Section 40 reproduces Section 46 of the Indian Evidence Act, 1872 (IEA) without any change in wording. The BPRD BSA-to-IEA correspondence table records "No change." Its role is subordinate to Section 39: it activates only when an expert opinion is relevant under that section, and it borrows its own relevance from it.
Facts otherwise irrelevant, made relevant here
The provision's clever twist is the phrase "not otherwise relevant". Facts about other people's reactions to a poison, or about other harbours far from the plaintiff's, would ordinarily fail the general relevancy tests in Chapter II. They are not the facts in issue, they do not form part of the same transaction, and they do not touch the parties before the court. Yet Section 40 makes them relevant precisely because they bear on an expert's opinion — either buttressing it or undermining it.
The reasoning is straightforward. Expert opinion often rests on generalisations from experience — the symptoms this poison typically produces, the effects a wall of a certain size tends to have on a particular kind of harbour. Facts that confirm or contradict those generalisations tell the court whether the expert's opinion is well-founded in the world outside this trial. Without Section 40, the trial would be starved of the very context that makes expert testimony testable.
Two directions, one function
Section 40 admits facts supporting an expert opinion and facts inconsistent with it, treating both symmetrically. That symmetry is important. It means the party leading expert opinion can strengthen it by producing corroborating facts, and the opposing party can undermine it by producing contradicting facts, without either having to reargue the relevancy question. The section takes both sides' hands off the relevancy dispute and delivers them onto the substantive one — is the expert opinion, tested against these external facts, sound or not?
The two illustrations, worked through
Illustration (a) is the classic poisoning example. The question in the case is whether A was poisoned by a certain poison. The fact that other people poisoned by that poison exhibited particular symptoms — symptoms that experts affirm or deny to be characteristic of that poison — is a fact about other persons in other times, unrelated to A. Ordinarily it would be irrelevant to A's case. Section 40 makes it relevant because it tests the expert's testimony about how the poison manifests.
Illustration (b) is the harbour example. The question is whether an obstruction to a harbour has been caused by a certain sea-wall. Facts about other harbours — similar in relevant respects but without such a sea-wall — beginning to be obstructed at about the same time are relevant. Those facts have nothing directly to do with this harbour or these parties. But they bear on the expert claim that this sea-wall causes obstruction. If sea-wall-free harbours are becoming obstructed too, the causation hypothesis is weakened; if only harbours with such walls are affected, it is strengthened.
Placement in the opinion cluster
| Section | Function |
|---|---|
| 39 | Admits expert opinion on foreign law, science, art, "any other field", handwriting/finger impressions, and electronic evidence |
| 40 | Admits otherwise-irrelevant facts if they bear on a relevant expert opinion |
| 41 | Admits lay opinion on handwriting/signature (and electronic signature via CA) |
| 42 | Admits opinion on general customs or rights |
| 43 | Admits opinion on usages, tenets, and local vocabulary |
| 44 | Admits opinion, expressed by conduct, on relationship |
| 45 | Admits the grounds on which any of these opinions rests |
How the section works with Section 45
Section 45 makes the grounds of a relevant opinion themselves relevant — the expert's reasoning, experiments, and materials. Section 40 does something related but distinct: it admits facts from outside the expert's own record that bear on that reasoning. The two provisions therefore work together but do not overlap. The expert's grounds enter under Section 45; corroborating or contradicting empirical facts from elsewhere in the world enter under Section 40. A party challenging an expert opinion is entitled to attack both — probing the internal grounds through Section 45 and adducing external contradictory facts through Section 40. The court, taking both streams into account, is far better placed to weigh the opinion than it would be if only the expert's own materials were before it.
Why the section survives unchanged
The BSA has substantially expanded Section 39 by adding "or any other field" — a scope-broadening change that lets experts in emerging areas be recognised. Section 40, however, was not touched. It did not need to be. Whatever field an expert speaks to under the enlarged Section 39, Section 40 automatically operates alongside: any fact bearing on that expert's opinion, whether supportive or inconsistent, becomes relevant. Widening Section 39 has, by that mechanism, silently widened the reach of Section 40 too.
Key Takeaways
- Companion to Section 39: Section 40 activates when an expert opinion is relevant and admits facts that bear on it.
- Both directions: facts supporting and facts inconsistent with an expert opinion are equally admissible.
- Otherwise-irrelevant facts count: facts about other persons, other harbours, other analogous situations may enter under this section.
- Unchanged from Section 46 IEA: the recodification kept the provision verbatim.
- Reach widens with Section 39: as experts in "any other field" are now admissible, so are the corroborating or contradicting facts around them.
Conclusion
Section 40 is a small provision doing outsized work. By loosening the relevancy test just enough to let corroborating and contradicting facts around an expert opinion enter the record, it turns expert testimony from a monologue into something the court can actually assess. Its two illustrations — the poison and the harbour — are as vivid today as they were in 1872, and they explain the section better than any abstract account. Unchanged from Section 46 IEA, it continues to sit at the practical heart of expert-evidence litigation, silently activated every time an expert takes the stand.