The Indian Evidence Act, 1872 has defined circumstantial evidence.
As per the act, for the conviction on the basis circumstantial evidence, the following conditions must be full filled:

• The circumstances from which the conclusion of the guilt is to be drawn should be fully established.
   
• The facts so established should be consistent not only with the hypothesis of the guilt of the accused, that is to say, they should not be explainable on any other hypothesis except that accused is guilty.
   
• The circumstances should be of conclusive nature and tendency
   
• They should exclude the possibility of every possible hypothesis except the one to be proved
   
• There must be a chain of evidence so complete as to not leave any reasonable ground for the conclusion consistent with the innocence of the accused and must show that in all human probability, the act must have been done by accused.

The concept of circumstantial evidence is usually used in the cases of criminal law and while pleading the same it is really essential to draw and link the series of events with the facts of the case to from the chain of circumstantial evidence. Facts which though not in issue, are so connected with a fact in issue as to form part of the same transaction, are relevant, whether they accused at the same time and place or at different times and places[1] plays an important role when it comes to forming the chain of circumstantial evidences.

There are a lot of cases with regards to this concept,[2]but one of the most landmark yet controversial case where the concept of doctrine of circumstantial evidences was used was the Aarushi Murder Case. The case establishes the chain of these evidences by making a direct analogy with the facts. An interesting part here is that the doctrine of last seen was used to form the chain of circumstantial evidences, 

The doctrine of last seen
Last seen together theory is one in which two people are ' seen together ' and one is found alive after an interval of time, and another is dead. Last seen theory may by itself be a poor kind of evidence establishing conviction on the same. Last seen together principle is one of the latest principles which is taken into consideration in establishing the guilt of the accused.

Doctrine of last seen, if proved, shifts the burden of proof onto the accused, placing on him the onus to explain how the incident occurred and what happened to the victim who was last seen with him. If there is a failure on part of the accused to furnish any explanation in this regard, as in the case in hand, or furnishing false explanation would give rise to a strong presumption against him, and in favour of his guilt, and would provide an additional link in the chain of circumstances[3],

As mentioned above the example of Aarushi murder case, this was the doctrine that the prosecution used with regards to the fact that four people in the house, i.e. the two accused and two victims were last seen in the house by their driver and after that there were no entry or exit signs of any outsider and on the basis of this fact and this doctrine the other chain of circumstantial evidences was framed.

When we are talking about circumstantial evidences and the doctrine of last seen, it is essential that Facts are necessary to explain or introduce a fact in issue or relevant fact, or which support or rebut an inference suggested by a fact in issue or relevant fact, or which establish the identity of anything or person whose identity is relevant, or fix the time or place at which any fact in issue or relevant fact happened, or which show the relation of parties by whom any such fact was transacted, are relevant in so far as they are necessary for that purpose[4] . The purpose and analogy of the three elements we are talking about puts the burden on defence with regards to criminal law cases.

When we are talking about relevancy of facts, it obviously plays an important role to from the chain of circumstantial evidences but another thing that is important is Facts showing existence of state of mind, or of body or bodily feeling.[5]

The article can be concluded on the fact that in criminal law the burden of proof lies on the prosecution and the tool of relevancy of facts, along which facts showing existence of state of mind or body can be used to form the chain of circumstantial evidence to leave no reasonable doubt of defence.

End-Notes:

1. Section 6, Indian Evidence Act
2. Sharad Birdhichand Sharda v. State (AIR 1984 SC 1622), Kusuma Ankama Rao v. State of A.P. (AIR 2008)
3. https://www.soolegal.com/roar/doctrine-of-last-seen-theory
4. Section 9, Indian Evidence Act, Facts necessary to explain or introduce relevant facts.
5. Section, Indian Evidence Act