The term opinion means a view or estimation. It means something more than mere gossiping or hearsay. It means judgment or belief, that is a belief or a conviction resulting from what one thinks on a particular question. The opinion differs from person to person. Even as the opinion of a person changes from time to time.

However, in certain occasions, the circumstances of the cases make it necessary to obtain the opinions of third parties. Generally, a court will not be able to form a correct opinion about the matter in issue. In such cases help of an expert is required because to form an opinion in such matters special study or training or experience is necessary.

Who is an Expert?

It simply means an expert is a person who has special knowledge and skill in any particular subject via observation, experience, study, practice to which inquiry relates. Section 45 of Indian Evidence Act further states that the person who is skilled and has special knowledge and experience in the following field is called experts.

• Foreign law
• Science
• Art
• Identity of Handwriting
• Identity of finger impression

Relevancy of Expert's opinion

Section 45 to 51 of the Evidence Act lay down the General Principles and procedures about opinion of third persons when relevant. The opinion of the following persons may be accepted by the court.

1. Opinion of experts (Sec. 45)
   As the Indian court is not supposed to be conversant with foreign law, expert's opinion is admitted. As science or art or identity of handwriting or finger impression has a broader aspect, expert's opinion on this matter is admitted.
    
2. Opinion of Examiner of Electronic Evidence (Sec 45-A)
   When a court has to form an opinion relating to any information transmitted or stored in any computer, electronic form, or digital form, the opinion of the examiner of such electronic evidence referred to sec 79A of the IT Act, 2000, who are called experts, is relevant.
    
3. Facts bearing upon opinions of experts (Sec. 46)
   In matters where the opinion of the expert is relevant, any other fact that supports or is inconsistent with that opinion, though not otherwise relevant, is relevant.Example: In Folkes v. Chad, the question at hand is whether a particular sea wall is responsible for blocking a harbor or not. The expert opinion of an engineer was that other harbors, located in similar ways in terms of their surroundings and conditions, started to experience blockages at around the same time but did not have sea walls. It suggests that something other than the sea wall might be causing the blockages. This information is relevant because it supports the argument that the sea wall may not be the sole or primary cause of the harbor blockages.
    
4. Opinion as to handwriting, when relevant (Sec. 47)
   When the court needs to form an opinion on a document that is either written or signed by a person, the opinion of any person who is acquainted with the handwriting of the person who was supposed to write/sign is a relevant fact.Section 47 says that a person is said to be acquainted with the handwriting of another person when he has:
   1. Personally witnessed someone writing something.
   2. Received written documents from that same person in response to documents they themselves wrote or authorized.
   3. In their ordinary course of business dealings, received documents from that person or documents that are habitually written and sent to them.
   Example: A and B do business. The question was whether a letter was written by A or not. B reads A's letter. C, a clerk of B, receives the letter of A. D, an employee of B, adds the letters to files. B reads A's letter. Here, the opinions of B, C, and D are relevant.
    
5. Opinion as to digital signature when relevant (Sec. 47-A)
   When the court has to form an opinion as to the digital signature of any person, the opinion of the certifying authority which has issued the digital signature certificate is a relevant fact.
    
6. Opinion as to the existence of right or custom, when relevant (Sec. 48)
   When the court has to form an opinion on whether any general custom or right does exist or not, then the opinion of such a person who has the knowledge of its existence is relevant.Example: If there are 100 villagers in a village, their opinion on any right or custom of that village is relevant.
    
7. Opinion as to usages, tenets, etc., when relevant (Sec. 49)
   Opinions of such a person who has special means of knowledge in respect of the following matters are relevant:
   1. Usages and tenets of any body of men or family,
   2. Constitution and government of any religious or charitable foundation, or
   3. The meaning of words or terms used in particular districts or by particular classes of people.
       
8. Opinion of relationship when relevant (Sec. 50)
   When the Court has to ascertain a question of relationship between two or more parties, the opinion of any person is relevant having:
   1. Special means of knowledge;
   2. Expressed by conduct
   Example: The question is whether A was the legitimate son of B. The fact that A was always treated as such by members of the family is relevant. But this opinion is not sufficient to prove marriage under the Indian Divorce Act, 1869, or marriage under offenses against marriage under IPC.
    
9. Grounds of opinion, relevant (Sec. 51)
   The grounds of a living person's opinion are also relevant.
   
   Example- Experiments for giving the opinion are relevant.

Evidentiary value of expert opinion:

As the expert opinion is a weak type of evidence it is usually considered to be of light value.

1. The evidence of an expert is not conclusive.
2. The opinion of the expert is not binding upon a judge, and that is why the court can refuse to rely on the evidence of an expert if it is not supported by circumstantial evidence.
3. No opinion of an expert is admissible unless he has been examined as a witness. The adverse party has a right and opportunity to cross-examine the expert. Before expert testimony can be admitted, "two things must be proved, viz.
• The subject is such that expert testimony is necessary. There are cases in which the court is not in a position to form a correct opinion, e.g., common knowledge or when a special study of a subject or special experience therein is necessary.
• That the witness in question is really an expert.

Case laws:
In S. Gopala Reddy v. State of Andhra Pradesh (1996), the Supreme Court held that a piece of expert evidence is a weak type of evidence. Courts do not consider it as conclusive and therefore, it is not safe to rely upon it without seeking independent and reliable corroboration.

In Darshan Singh v. State of Haryana (1997), Supreme Court held that where there is inconsistency between eye witnesses on the point of how the injury was caused, the evidence of the doctor cannot override the unimpeachable testimony of the eye witness. [Evidence of doctor cannot override testimony of eye witness]

Difference between medical evidence and ocular evidence:
In Palani v. State of Tamil Nadu, (2020) Supreme Court held that if there is a variance between medical evidence and ocular evidence, the ocular evidence must be given primacy. Medical evidence is basically opinionative.

In Bal Krishna Das Agrawal v. Radha Devi and Others (1989), it was held that an expert is a person who by his training and experience has acquired the ability to express an opinion that an ordinary witness does not process. The evidence of an expert is such evidence that is based on expertise and experience

In Abdul Rahman v. State of Mysore (1972), The opinion of a professional goldsmith as to the purity of the gold in question was held to be relevant as the opinion of an expert, though he had no formal qualifications. His only qualification was experience.

Conclusion
From the above analysis it may be submitted that evidence of an expert is not a substantive piece of evidence. The courts do not consider it conclusive. Without independent and reliable corroboration it may have no value in the eye of law. Once the court accepts an opinion of an expert, it ceases to be the opinion of the expert and becomes the opinion of the court.