Expert and Opinion Evidence under Sections 39 to 45 of the Bharatiya Sakshya Adhiniyam, 2023: Expert Opinion, Medical, Forensic, DNA, Handwriting and Electronic Evidence

A witness deposes to what he perceived. What he thinks about it is ordinarily no business of the court, whose function it is to draw the inferences. Sections 39 to 45 set out the situations in which that rule gives way — where the question requires knowledge the court does not possess, or where the fact to be proved is of a kind that only those familiar with it can speak to. The Adhiniyam has widened the first of these considerably, by adding a residuary field to the list of expert subjects and by giving the Examiner of Electronic Evidence a sub-section of his own.

1. The General Rule and Why It Yields

The rule excluding opinion rests on two ideas. The first is that the tribunal decides the case, and a witness who offers his conclusion is doing the tribunal's work. The second is that an opinion cannot be tested in the way a perception can: a witness who says he saw something can be asked where he stood and how the light was, but a witness who says he believes something can only be asked why.

Both ideas break down where the question lies outside ordinary experience. A judge cannot determine whether a wound was caused by a blunt or a sharp weapon, whether two signatures were written by one hand, whether a firearm discharged the recovered cartridge, or whether a biological sample matches another. In such cases the court must either receive the opinion of somebody who can, or decide the question by guesswork. Sections 39 to 45 choose the first.

The group also admits opinion of a second kind, which has nothing to do with expertise. Whether a person wrote a particular letter, whether a custom exists, and whether two people are related are facts known to those familiar with the person, the locality or the family. Sections 41 to 44 admit their opinion because familiarity, not skill, is what the question requires.

2. Section 39 — Expert Opinion

Section 39, BSA — Opinions of experts

(1) When the Court has to form an opinion upon a point of foreign law or of science or art, or as to identity of handwriting or finger impressions, the opinions upon that point of persons specially skilled in such foreign law, science or art, or in questions as to identity of handwriting or finger impressions, or in any other field, are relevant facts. Such persons are called experts.

(2) When in a proceeding, the Court has to form an opinion on any matter relating to any information transmitted or stored in any computer resource or any other electronic or digital form, the opinion of the Examiner of Electronic Evidence referred to in section 79A of the Information Technology Act, 2000, is a relevant fact.

Section 39 corresponds to Sections 45 and 45A of the Indian Evidence Act, consolidated into one section with two sub-sections. Two changes are worth noting.

The first is the addition of the words 'or in any other field'. Section 45 of the earlier Act listed foreign law, science, art, handwriting and finger impressions, and the courts had to strain the word 'science' to accommodate disciplines that plainly were not in contemplation in 1872. The residuary field removes the difficulty and brings within the section any body of specialised knowledge — cyber forensics, accountancy, valuation, engineering, linguistics, and whatever comes next.

The second is the elevation of the Examiner of Electronic Evidence to a sub-section of his own. Section 45A of the Indian Evidence Act, inserted in 2008, said the same thing, but placing it as Section 39(2) alongside the general provision has produced a question of construction that the Supreme Court has now answered.

📖 Pune Bar Assn. v. Union of India, 2026 SCC OnLine SC 1297 (decided 22 May 2026)

Held: A three-Judge Bench (Surya Kant, CJI, Joymalya Bagchi and Vipul M. Pancholi, JJ.) upheld the validity of Section 63(4) of the Adhiniyam and the Schedule. Reading Sections 39(1) and 39(2) harmoniously, the Court held that the 'expert' who signs Part B of the certificate is not confined to an Examiner of Electronic Evidence notified under Section 79A of the Information Technology Act, 2000. Any person possessing special skill and expertise in computer science or cyber forensics may sign, provided the court is satisfied of the credentials on unimpeachable material.

Ratio: Section 39(2) does not create a monopoly. The residuary words in Section 39(1) leave the general category of expert open, and a person skilled in cyber forensics is an expert whether or not he holds a notification under Section 79A.

3. Who Is an Expert

The section says 'persons specially skilled'. It does not say qualified, certified or employed by the State, and there is no requirement of a degree. What must be shown is that the witness has acquired, by study, training or long practical experience, knowledge of the subject that the court does not have.

The party tendering the witness must lay this foundation before the opinion is received. In practice it is done at the start of examination-in-chief, by taking the witness through his qualifications, training, experience, the number of comparable examinations he has conducted and any published work. The opposing party may challenge the foundation, and where it is not laid, the opinion is not receivable at all — a point frequently overlooked, with the result that a report is admitted and later found to be worthless because nobody established that its author was competent to write it.

📖 Ramesh Chandra Agrawal v. Regency Hospital Ltd., (2009) 9 SCC 709

Held: The scope of expert evidence is that where the court has to form an opinion on a subject requiring special skill, the opinion of a person specially skilled is relevant. But the credibility of such a witness depends on the reasons stated in support of his conclusions and the data and material furnished which form the basis of his conclusions. The court is not bound by the evidence of experts, which is to a large extent advisory in nature. An expert deposes and does not decide, and his duty is to furnish the judge with the necessary scientific criteria for testing the accuracy of his conclusions so as to enable the judge to form his own independent judgment.

Ratio: The governing statement on the function of an expert. He supplies the criteria; the court forms the judgment.

📖 State of Himachal Pradesh v. Jai Lal, (1999) 7 SCC 280

Held: An expert is not a witness of fact and his evidence is really of an advisory character. The duty of an expert witness is to furnish the judge with the necessary scientific criteria for testing the accuracy of the conclusions, so as to enable the judge to form his own independent judgment by the application of these criteria to the facts proved in evidence. An expert who merely states his conclusion without giving reasons or the data on which it rests is of no assistance, and his evidence cannot be acted upon. The court must satisfy itself of the expert's competence and of the soundness of his reasoning.

Ratio: A bare conclusion is not expert evidence. Reasons and data are what make it usable.

4. Advisory, Not Conclusive

The settled position, repeated in every decision on the subject, is that expert evidence is advisory. It is admitted to assist the court, not to displace it. The court is not bound to accept it, and it may prefer the evidence of ordinary witnesses over the opinion of an expert.

Four consequences follow, and they govern the practical handling of such evidence.

The court must apply its own mind. A judgment that recites an expert's conclusion and adopts it without examining the reasoning has abdicated the function the expert exists to assist.
The reasoning is more important than the conclusion. An opinion supported by stated criteria and data can be tested; one that is not, cannot, and is of no value however eminent its author.
The expert must ordinarily be examined. A report is not evidence merely by production, and unless a statutory provision permits it to be received without examination, the author must be called and made available for cross-examination.
Expert evidence is ordinarily corroborative. It supports the case established by other material rather than founding it, and courts are reluctant to convict on expert opinion alone in most fields.

⚠ Expert evidence and the burden of proof

An expert's opinion never shifts the burden. Where the prosecution relies on a forensic report, it must still prove that the sample examined was the sample seized, that the chain of custody was unbroken, and that the article is connected with the offence. A favourable report on an article whose provenance has not been established proves nothing at all, and a great many forensic cases are lost on custody rather than on science.

5. Medical Evidence

Medical evidence is the most frequently encountered form of expert evidence in criminal trials. It establishes the cause of death, the nature and situation of injuries, the weapon that could have caused them, the approximate time of death, the presence of poison or intoxicants, and the age of a person.

5.1 The post-mortem report

A post-mortem report is the record of the examination conducted on a body. Two propositions about it are settled and are frequently confused.

The report is not substantive evidence. It is a document recording what the medical officer observed and concluded, and it becomes evidence when he deposes to it. Where the medical officer is dead, has become unavailable, or has been succeeded in office, the position is governed by the provisions of the criminal procedure legislation permitting reports of certain classes of expert to be used without examination, and by Section 26(b) of the Adhiniyam where the maker is dead and the report was made in the discharge of professional duty.

The opinion in the report is relevant under Section 39; the observations in it are the medical officer's account of what he saw. The distinction matters because the observations — the number, situation and dimensions of the injuries — are matters of fact within his perception, while the conclusions he draws from them are opinion. An opinion may be displaced by better reasoning; an observation must be displaced by showing that he did not see what he says he saw.

5.2 Medical evidence and ocular evidence

The relationship between what the eyewitnesses say and what the doctor says is one of the most litigated questions in criminal practice, and the principle is settled.

📖 Solanki Chimanbhai Ukabhai v. State of Gujarat, (1983) 2 SCC 174

Held: Ordinarily the value of medical evidence is only corroborative. It proves that the injuries could have been caused in the manner alleged and nothing more. The use to which such evidence can be put is that it can be used to show that the injuries could not possibly have been caused in the manner alleged, and thereby discredit the eyewitnesses. Unless, however, the medical evidence in its turn goes so far that it completely rules out all possibility of the ocular evidence being true, the ocular testimony of eyewitnesses is not discredited.

Ratio: The governing statement. Medical evidence corroborates; it displaces ocular testimony only where it rules out the possibility of that testimony being true.

📖 Abdul Sayeed v. State of Madhya Pradesh, (2010) 10 SCC 259

Held: Where the medical evidence totally improbabilises the ocular version, or is at total variance with it, it may be considered a factor for discrediting the eyewitnesses. But where the medical evidence merely suggests an alternative possibility, and does not exclude the version deposed to, the ocular evidence prevails. The evidence of a witness of fact is not to be discarded on the ground that a medical opinion suggests a different possibility, since medical opinion is itself a matter of degree.

Ratio: The threshold is total improbability, not mere inconsistency.

The practical working of this is that a minor discrepancy between the eyewitness account and the medical findings — as to the exact number of blows, or the precise weapon among several of the same class — does not defeat the prosecution. What defeats it is a contradiction of a different order: an account of stabbing where the injuries are entirely blunt, or of a shooting at close range where there is no blackening or tattooing.

6. Forensic and Scientific Evidence

6.1 DNA evidence

DNA profiling has become the most powerful form of identification evidence available, and its treatment illustrates the tension between scientific certainty and legal presumption.

📖 Nandlal Wasudeo Badwaik v. Lata Nandlal Badwaik, (2014) 2 SCC 576

Held: Where a DNA test has in fact been conducted and its result excludes paternity, the result must prevail. When there is a conflict between a conclusive proof envisaged by law and a fact established by science, the latter must prevail, since the presumption under Section 112 of the Indian Evidence Act is a legal fiction whereas the DNA report is a matter of scientific accuracy. The Court nevertheless reiterated that such a test is not to be directed as a matter of routine.

Ratio: A reliable DNA result excluding paternity displaces even a conclusive statutory presumption.

📖 Goutam Kundu v. State of West Bengal, (1993) 3 SCC 418

Held: Courts in India cannot order a blood test as a matter of course. There must be a strong prima facie case, and the applicant must establish the need for such a test. No one can be compelled to give a sample of blood for analysis, and an adverse inference may be drawn against a party who refuses. The presumption under Section 112 is a conclusive presumption which can be displaced only by proof of non-access.

Ratio: The test will rarely be ordered; read with Nandlal Badwaik, the working position is that where a reliable result exists it governs, but the order for testing is not made lightly.

📖 Sharda v. Dharmpal, (2003) 4 SCC 493

Held: A matrimonial court has the power to order a person to undergo a medical examination, and passing such an order would not be in violation of the right to personal liberty under Article 21. The court should exercise the power only if the applicant has a strong prima facie case and there is sufficient material before it. If, despite the order, the person refuses to submit to the examination, the court will be entitled to draw an adverse inference against him.

Ratio: The court has the power to order examination in matrimonial proceedings, but it is exercised on strong material, and refusal carries a consequence.

Two practical points apply to all DNA evidence. The chain of custody of the sample must be established without a gap, from collection through sealing, transmission and storage to examination, because a result relating to a sample that cannot be connected to the accused proves nothing. And the methodology must be deposed to, since the probative force of a match depends on the number of loci examined and the statistical frequency of the profile in the relevant population.

6.2 Ballistic evidence

Where a firearm is alleged to have been used, ballistic evidence connects the recovered weapon to the recovered projectile or cartridge case. The examination compares the striations left by the barrel and the marks left by the firing pin and breech face, and these are as individual as a fingerprint where the comparison is properly conducted.

📖 Sukhwant Singh v. State of Punjab, (1995) 3 SCC 367

Held: Where a firearm is alleged to have been used, it is the duty of the prosecution to examine the ballistic expert and to prove his opinion connecting the weapon with the crime. The failure to produce the expert opinion before the trial court, where the weapon has been recovered and sent for examination, affects the creditworthiness of the prosecution case to a great extent. The court cannot assume a connection between the recovered weapon and the injuries in the absence of such evidence.

Ratio: Where a weapon has been recovered and examined, the expert must be produced. An unexplained failure to do so tells against the prosecution.

6.3 Fingerprint evidence

Finger impressions are expressly named in Section 39(1), and the science is among the oldest and most firmly established forms of identification. A comparison properly conducted, with an adequate number of matching ridge characteristics and no unexplained differences, is treated as reliable.

The value of the evidence depends almost entirely on the integrity of the lifting and comparison process. What must be established is where the print was lifted from, when, by whom, that the surface was one the accused had no innocent occasion to touch, how the specimen prints were taken and by whom, and the points of comparison relied upon. Where the print is said to have been found on a portable article, or on a surface the accused might lawfully have touched, its evidentiary value falls away sharply, since it establishes contact and not participation.

7. Handwriting Evidence

Handwriting may be proved in four distinct ways, and it is important to keep them apart because their evidentiary weight differs greatly.

Route	Provision	Who deposes	Weight
Direct evidence	Sections 54 and 55	A person who saw the document written or signed	Highest, subject to credibility
Admission	Sections 15 to 21	The writer himself, admitting the writing	Very high; substantive evidence against him
Opinion of a person acquainted with the handwriting	Section 41(1)	A person who has himself seen the party write, or received documents purporting to be written by him in answer to his own communications, or in the ordinary course of business has had documents habitually submitted to him	Moderate; depends on the extent of familiarity
Expert opinion	Section 39(1)	A person specially skilled in identifying handwriting	Weakest of the four in practice; corroboration ordinarily sought
Comparison by the Court	Section 72	The court itself, comparing the disputed writing with writing admitted or proved to be genuine	A supplementary route; used with caution

7.1 The expert and the acquainted person

Section 41, BSA — Opinion as to handwriting and electronic signature, when relevant

(1) When the Court has to form an opinion as to the person by whom any document was written or signed, the opinion of any person acquainted with the handwriting of the person by whom it is supposed to be written or signed, that it was or was not written or signed by that person, is a relevant fact.

Explanation. — A person is said to be acquainted with the handwriting of another person when he has seen that person write, or when he has received documents purporting to be written by that person in answer to documents written by himself or under his authority and addressed to that person, or when, in the ordinary course of business, documents purporting to be written by that person have been habitually submitted to him.

(2) When the Court has to form an opinion as to the electronic signature of any person, the opinion of the Certifying Authority which has issued the Electronic Signature Certificate is a relevant fact.

The Explanation is exhaustive of the ways in which acquaintance may be acquired, and this is often overlooked. A witness who says merely that he is 'familiar with' the writing, without bringing himself within one of the three modes, has not established the foundation the section requires.

7.2 The weakness of handwriting expert evidence

📖 Magan Bihari Lal v. State of Punjab, (1977) 2 SCC 210

Held: It is unsafe to base a conviction solely on the opinion of a handwriting expert. Such opinion evidence is of a frail character and is hedged about with a great many uncertainties. Before acting upon it, the court must seek independent and reliable corroboration.

Ratio: The classical statement of the caution with which handwriting expert evidence is treated.

📖 Murari Lal v. State of Madhya Pradesh, (1980) 1 SCC 704

Held: There is no rule of law, nor any rule of prudence which has crystallised into a rule of law, that the opinion of a handwriting expert must never be acted upon unless substantially corroborated. But the imperfections of the science and the risk of error are such that the approach should be one of caution, and reasons for the opinion must be carefully probed and examined. Where the reasons are convincing and there is no reliable evidence throwing doubt on them, there is no reason to distrust the opinion. The hazard in accepting such opinion is not because it is unreliable but because human judgment is fallible.

Ratio: The corrective to Magan Bihari Lal. Caution is required, but there is no absolute rule against acting on a well-reasoned handwriting opinion.

The two decisions are read together in practice. A handwriting opinion supported by a clear statement of the characteristics compared, illustrated by enlargements, and unshaken in cross-examination may be acted upon; one that states a conclusion without demonstrating how it was reached will not be. The requirement of reasons stated in Jai Lal operates here with particular force.

7.3 Comparison by the court

Section 72 permits the court itself to compare a disputed writing, signature or seal with any writing, signature or seal admitted or proved to be genuine. The power is real and is exercised, but it is exercised with caution: a judge is not a handwriting expert, and where the question is genuinely contested the court will ordinarily prefer to have the assistance of one. The power is most useful where the difference is gross, or where it confirms an opinion already given.

8. Electronic Evidence and the Examiner

Section 39(2) makes relevant the opinion of the Examiner of Electronic Evidence referred to in Section 79A of the Information Technology Act, 2000. That provision empowers the Central Government to notify any department, body or agency as an Examiner of Electronic Evidence for the purpose of providing expert opinion on electronic form evidence.

The practical difficulty has been the small number of notified Examiners relative to the volume of electronic evidence now tendered, and the question whether a person skilled in cyber forensics but not notified could give an opinion, or sign the expert declaration in Part B of the certificate under Section 63(4). That question is now settled.

Reading Sections 39(1) and 39(2) harmoniously, the Supreme Court has held that the general category of expert in sub-section (1) — widened by the residuary words 'or in any other field' — is not displaced by the specific provision in sub-section (2). A person possessing special skill in computer science or cyber forensics is an expert, and may give an opinion and sign Part B, provided the court is satisfied of his credentials on unimpeachable material.

⚠ Opinion and proof are separate questions

Section 39(2) makes the Examiner's opinion relevant. It says nothing about how the electronic record he examined is to be proved, which is governed by Sections 57, 61 and 63 and the Schedule. A forensic report on a device is of no use if the underlying record cannot be proved, and a properly certified record does not become reliable because an expert has commented on it. The two requirements must be satisfied independently, and failure at either point defeats the evidence.

9. Section 40 — Facts Bearing Upon Expert Opinion

Section 40, BSA — 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.

Section 40 is a small provision of considerable practical use. Once an expert opinion has been received, facts that would otherwise be irrelevant become relevant if they support or contradict it. The section is the route by which the machinery of testing expert evidence enters the record.

The material commonly admitted under it includes published literature contradicting the opinion, the results of other examinations of the same material, the expert's own earlier opinions in comparable cases, the conditions in which the examination was conducted, the state and adequacy of the sample, and the accepted standards of the discipline. None of these is relevant to the fact in issue; all of them bear on whether the opinion can be accepted.

10. Sections 42, 43 and 44 — Opinion Founded on Familiarity

Three sections admit opinion of a kind that has nothing to do with expertise. What they rest on is familiarity with a locality, a body of persons, or a family.

10.1 Section 42 — General custom or right

Section 42, BSA — Opinion as to existence of general custom or right, when relevant

When the Court has to form an opinion as to the existence of any general custom or right, the opinions as to the existence of such custom or right, of persons who would be likely to know of its existence if it existed, are relevant.

Explanation. — The expression 'general custom or right' includes customs or rights common to any considerable class of persons.

The section is confined to a general custom or right, which the Explanation extends to customs common to any considerable class of persons. A purely private right is outside it and must be established under Section 11. The witness need not be an expert; he must be a person who would be likely to know — an old inhabitant, a village elder, a person long connected with the community or the trade.

Section 42 is the living-witness counterpart of Section 26(d), which admits the opinion of a person now dead. The two differ in one further respect: Section 26(d) requires that the opinion have been expressed before any controversy arose, while Section 42 contains no such condition, because a living witness is available to be cross-examined about his interest.

10.2 Section 43 — Usages and tenets

Section 43, BSA — Opinion as to usages, tenets, etc., when relevant

When the Court has to form an opinion as to — the usages and tenets of any body of men or family; the constitution and government of any religious or charitable foundation; or the meaning of words or terms used in particular districts or by particular classes of people — the opinions of persons having special means of knowledge thereon are relevant facts.

Section 43 covers three distinct subjects. Usages and tenets of a body of persons or a family are relevant in disputes about religious practice, community obligations and family custom. The constitution and government of a religious or charitable foundation arises constantly in litigation about temples, mosques, churches, maths and trusts — who may manage, how successors are appointed, what the terms of the endowment are. The meaning of words or terms used in a particular district or by a particular class covers trade usage, local terminology and the specialised vocabulary of a calling.

Where Section 42 requires a person 'likely to know', Section 43 requires special means of knowledge, which is a higher standard. A priest of long standing, a hereditary trustee, a person who has practised the trade for decades — each qualifies; a casual observer does not.

10.3 Section 44 — Relationship

Section 44, BSA — Opinion on relationship, when relevant

When the Court has to form an opinion as to the relationship of one person to another, the opinion expressed by conduct, as to the existence of such relationship, of any person who, as a member of the family or otherwise, has special means of knowledge on the subject, is a relevant fact.

Provided that such opinion shall not be sufficient to prove a marriage in proceedings under the Divorce Act, 1869 or in prosecutions for offences relating to marriage under the Bharatiya Nyaya Sanhita, 2023.

The distinctive feature of Section 44 is the phrase 'expressed by conduct'. What is admitted is not what the witness says he believes but what somebody's behaviour shows that he believed. Treating a person as a son, giving him a share, performing the ceremonies proper to that relationship, describing him as such in documents over many years — each is conduct expressing an opinion, and the section makes it relevant.

📖 Dolgobinda Paricha v. Nimai Charan Misra, AIR 1959 SC 914

Held: The word 'opinion' in the provision corresponding to Section 44 means something more than mere retailing of gossip or hearsay; it means a judgment or belief, that is, a belief or conviction resulting from what one thinks on a particular question. The opinion must be founded on the special means of knowledge which the person had, and it is the conduct by which the opinion is expressed that the section makes relevant. Evidence of general reputation, unconnected with any conduct of a person having special means of knowledge, is not within the section.

Ratio: The leading authority. A genuine belief founded on special knowledge, expressed by conduct, is what the section admits — not neighbourhood talk.

The proviso is important and is frequently missed. Opinion evidence under Section 44 is not sufficient to prove a marriage in matrimonial proceedings under the Divorce Act, 1869 or in prosecutions for offences relating to marriage. In those proceedings the marriage must be strictly proved by evidence of the ceremony or by the statutory record, because the consequences of the finding are penal or affect status directly.

11. Section 45 — Grounds of Opinion

Section 45, BSA — 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.

Section 45 is the shortest provision in the group and in some ways the most important. It makes the reasoning behind an opinion relevant, and it is the statutory basis of the entire practice of examining an expert about how he reached his conclusion.

Its significance follows from the advisory character of expert evidence. If the court is to form its own judgment, it must have the criteria on which the opinion rests. Section 45 makes those criteria relevant, Section 40 makes facts supporting or contradicting them relevant, and the requirement stated in Jai Lal that an expert must furnish reasons is the practical consequence of both.

The section applies to any living person whose opinion is relevant, and is therefore not confined to experts under Section 39. The grounds of an opinion under Sections 41 to 44 are equally relevant — how the witness became acquainted with the handwriting, what he has observed of the custom, what conduct of the family he has seen.

12. The Group in Outline

Section	Subject	What the witness must have
39(1)	Foreign law, science, art, handwriting, finger impressions, or any other field	Special skill in the field
39(2)	Information transmitted or stored in a computer resource or electronic form	To be an Examiner of Electronic Evidence under Section 79A, IT Act, 2000 — though Section 39(1) leaves the general category open
40	Facts supporting or inconsistent with an expert opinion	Not applicable — the section admits facts, not opinion
41(1)	Handwriting and signature	Acquaintance with the handwriting, acquired in one of the three ways in the Explanation
41(2)	Electronic signature	To be the Certifying Authority which issued the Electronic Signature Certificate
42	General custom or right	To be a person likely to know of its existence
43	Usages and tenets; constitution of a religious or charitable foundation; meaning of local or trade terms	Special means of knowledge
44	Relationship, expressed by conduct	Special means of knowledge, as a family member or otherwise
45	Grounds of any relevant opinion	Applies to any living person whose opinion is relevant

13. The Position Stated Shortly

Opinion is excluded as a general rule, and Sections 39 to 45 are the exceptions.
Section 39(1) now carries a residuary field, so that any body of specialised knowledge qualifies and the word 'science' need no longer be strained.
Section 39(2) does not create a monopoly. A person skilled in cyber forensics is an expert under Section 39(1) whether or not he is notified under Section 79A of the IT Act.
Expert evidence is advisory. The expert deposes and does not decide, and his duty is to furnish the criteria by which the court may form its own judgment.
A bare conclusion is not expert evidence. Reasons and data are what make an opinion usable, and an opinion without them cannot be acted upon.
Medical evidence is ordinarily corroborative, and displaces ocular testimony only where it completely rules out the possibility of that testimony being true.
Handwriting expert evidence is approached with caution but is not subject to any absolute rule requiring corroboration.
Sections 42, 43 and 44 rest on familiarity, not skill, and Section 44 admits opinion expressed by conduct rather than by words.
Section 45 makes the grounds of any relevant opinion relevant, and is the statutory basis of the whole practice of testing an expert.

14. Related Topics and Provisions

Topic or provision	Connection
Substantive Evidence and Corroborative Evidence	The status of expert opinion as corroborative in most fields
Direct Evidence vs Circumstantial Evidence	Medical and forensic evidence as circumstances in a circumstantial case
Statements Relating to Public Right or Custom — Section 26(d)	The counterpart of Section 42 for persons now dead
Statements Regarding Relationship and Pedigree — Section 26(e) and (f)	The counterpart of Section 44 for persons now dead
Presumptions under the Law of Evidence	Section 116 on legitimacy, and its relationship with DNA evidence
Sections 57, 61 and 63 and the Schedule, BSA	Proof of electronic records, distinct from the opinion of an Examiner
Section 72, BSA	Comparison of signature, writing or seal by the court
Section 79A, Information Technology Act, 2000	Examiner of Electronic Evidence