Basically physical evidence is based on certain principles. These are follows;
Principle of individuality:
Every object , natural or human made , has an individuality, which is not duplicated in any other object. It is unique. Neither the nature has not duplicate itself nor human can
This principle is contrary to common belief and observation. The individuality has been varied in certain fields. The most extensive work has been carried out in finger print. Millions of fingerprints have been checked but on two fingerprints, even from two fingers of the same person have been ever found to be identical. The law of individuality is fundamental importance of physical evidence in forensic science. Anything and everything involved in a crime, has individuality. Thus the culprit is unique, scene of crime is unique , his/her modus operandi is unique, evidentiary clues left over or picked up by the culprit are unique. And this uniqueness in different person to person, through the physical evidence , the criminal can be arrest by linking the crime with criminal.
2. Principle of Exchange:
Whenever two entities came in contact, there is exchange of traces mutually. This principle says that, when a criminal or his/her instrument of crimes come in contact with the victim or the object surrounding him/her, they leaves traces. Thus, mutual exchange of traces takes place between criminal, the victim and the object involved in rime. The rationality behind this principle is the 3rd motion of Newton. Which states that “in every action, equal and opposite reaction”.[Ibid].According to this principle , when criminal attacks upon the victim by own self or by the help of object , the body or the victims any thing re-act in same motion and at the time the traces is exchanged between each other. The traces of victims go on the criminals and the traces of the offenders remains in the body of the victim. By that traces the offender can be find out easily.
3. Principle of Progressive Change:
Everything changes with the passage of time. Nothing in this world is unchangeable, permanent or invariable. But the rate of change varies with tremendously with different objects. The criminal undergo progressive change. If he/she is not apprehended in time , that becomes unrecognizable. But even in this , some evidences are those types which never changes in their lifetime. Such as fingerprints, bone fractures, or other characteristics of the permanent nature like body fluids, body cell, DNA profile never exchange. So this principle demands that prompt action in all aspect of criminal investigation.
4. Principle of Comparison:
Only the likes can be compare. This principle governs the physical evidence is comparison of the similar things. According to this principle , the evidence found in scene of crime is compared with the similar things before he/she had done. Generally it emphasizes the necessity of providing like sample and specimens for comparison with the question item. Under this principles the evidence found in scene of crime is compared with that’s previously done thing, and from this comparison that document is produced before the court as evidence. In this comparison the hand writing and printed document can compare to convict the criminal.
5. Fact do not lie:
Fact itself remain at any circumstances. Physical evidences are directly connected with the scene of crime and which is itself proving the fact that presence of criminal in that crime. So physical evidences can not lie the fact, and there is no room for error in proving or disproving the fact. The unique mark left by the offender remains only on him/her , so to prove the fact physical evidences are the best evidence.