An opinion, standing on its own, tells a court very little. What lets the court weigh an opinion, accept it, or reject it is the reasoning behind it. Section 45 of the Bharatiya Sakshya Adhiniyam, 2023 (BSA) recognises that principle: whenever the opinion of a living person is relevant, the grounds on which the opinion is based are also relevant.
45. Grounds of opinion, when relevant. Whenever the opinion of any living person is relevant, the grounds on which such opinion is based are also relevant.
Illustration. An expert may give an account of experiments performed by him for the purpose of forming his opinion.
Section 45 reproduces Section 51 of the Indian Evidence Act, 1872 (IEA) without any change in wording. The BPRD BSA-to-IEA correspondence table records "No change". Its position at the tail of the opinion-evidence cluster in Chapter II — after Sections 39 to 44 — is deliberate: it applies to every kind of relevant opinion the earlier sections admit, from expert opinion on foreign law or science under Section 39 to opinion on relationship under Section 44.
What the section actually admits
Where an opinion is itself relevant under an earlier section, Section 45 makes the "grounds" of that opinion equally relevant. The grounds are the reasoning, observations, materials, experiments, and premises on which the opinion rests. An expert who has concluded that a particular chemical caused a fatality may explain the tests conducted, the samples analysed, and the reasoning that led from data to conclusion. A person acquainted with a party's handwriting who has expressed an opinion under Section 41 may explain how they came to be acquainted — through correspondence, business dealings, or having seen the person write.
The illustration is short and pointed: "An expert may give an account of experiments performed by him for the purpose of forming his opinion." It captures the section's core purpose — the opinion enters the record with its supporting reasoning, not as a bare conclusion.
Why the grounds are indispensable
An opinion without grounds is closer to assertion than to evidence. A court called upon to accept an expert's view on cause of death or a lay witness's view on relationship needs the reasoning behind that view for three reasons. First, the reasoning is what allows the court to test whether the opinion is scientifically or logically sound. Second, it lets the opposing party cross-examine effectively — probing methodology, questioning assumptions, and exposing gaps. Third, the reasoning helps the court decide the weight to give the opinion, since two experts of equal experience may reach opposite conclusions on the same problem and only their grounds will reveal which analysis to prefer.
Reach across the opinion cluster
Section 45 is a support provision that borrows relevance from the sections around it. Its reach depends on which opinion is being led:
| Section | Opinion admitted | Grounds admissible under Section 45 |
|---|---|---|
| 39 | Expert opinion on foreign law, science, art or any other field | Methodology, tests, data, experiments, comparative material |
| 40 | Facts supporting or inconsistent with expert opinion | Analytical reasoning connecting facts to opinion |
| 41 | Opinion on handwriting/signature or electronic signature | Basis of acquaintance with the person or with the signature certificate |
| 42 | Opinion on general custom or right | Sources of knowledge in the community |
| 43 | Opinion on usages, tenets, meaning of local words | Special means of knowledge relied on |
| 44 | Opinion on relationship, expressed by conduct | The specific conduct on which the opinion is based |
The "living person" limitation
The section is expressly confined to opinions of a "living person". Where the opinion under consideration is contained in a treatise by a person who is deceased or otherwise unavailable, the mode of proof is governed by the special rule in Section 55 (second proviso, in effect the successor of the treatise-admissibility carve-out) rather than by Section 45. This distinction is not a technicality — it reflects the fact that a living witness can be cross-examined on their grounds, while a deceased author's grounds must instead be inferred from the treatise itself.
Cross-examination on grounds
Because Section 45 makes grounds relevant, they are legitimately the subject of cross-examination. An opposing party need not confine questioning to the ultimate opinion; they can and generally must probe the reasoning. In expert evidence, this is where cases are often won or lost: an expert whose methodology is flawed, whose data is incomplete, or whose reasoning is inconsistent with the record can be exposed only through examination of the grounds Section 45 makes admissible.
Practical dimensions in modern expert practice
The reach of Section 45 has grown as expert evidence has become more diverse. Forensic pathologists, digital forensic examiners, valuers, medical specialists, ballistics experts, DNA analysts, and now, under Section 39 as expanded to "any other field", experts in emerging areas — each brings a technical opinion that is only useful to a court to the extent its grounds can be laid out and tested. In many trials the expert's report is filed under procedural rules well before the witness enters the box; that report is essentially a written statement of grounds. Section 45 is what ensures that when the witness gives oral evidence, they may explain the same grounds fully, and it is what makes the report itself something the court can inspect alongside the testimony rather than treating either as a bare conclusion.
For a court, applying Section 45 well means insisting that opinion witnesses articulate their grounds clearly — the tests done, the data relied on, the assumptions made, and the reasoning applied — rather than presenting a naked bottom line. For counsel, it means preparing witnesses to explain their reasoning and, on the other side, preparing cross-examination that dismantles that reasoning where it is weak. Section 45 is the rule that makes both possible.
Key Takeaways
- Grounds follow the opinion: whenever a living person's opinion is relevant, the grounds on which it rests are also relevant.
- Unchanged from Section 51 IEA: the recodification carried the rule over verbatim.
- Applies across the opinion cluster: supports every opinion admitted under Sections 39 to 44.
- Essential for judicial assessment: per Ramesh Chandra Agrawal, the court must weigh, not defer to, expert opinion — grounds make that weighing possible.
- Only for living opinion-holders: deceased authors' opinions are handled elsewhere.
Conclusion
Section 45 closes the opinion-evidence cluster with a rule that gives the earlier sections their teeth. Admitting an opinion without admitting its grounds would leave the court with a conclusion it could neither test nor weigh. By making the reasoning relevant alongside the conclusion, the section enables real judicial engagement with opinion evidence — from expert reports to lay opinion on handwriting or relationship. Reproduced from Section 51 of the 1872 Act without change, it remains the quiet but essential companion to every opinion the BSA admits.