Reasonable doubt is a legal standard of proof necessary for validating a criminal conviction. Principle of reasonable doubt has higher leverage than the balance of probabilities; observed in civil matters; therefore, it is often used in criminal matters where someone's liberty is at more serious stake. The burden of proof in criminal matters is on the prosecution, therefore for a defendant to be found guilty the prosecution has to prove that the defendant is guilty of the crime he is charged with, in such a way that it remains any apprehension of reasonable doubt in the mind of the jury.
The doctrine is usually criticized as well for having a circular definition. Hence, the jury uses supplemental measures such as specific jury directions while deciding upon a case.
Reminisce can be found between Blackstone's theory and the principle of reasonable doubt that "it is better that ten guilty persons escape than that one innocent suffer", i.e. if there is any doubt that a person is guilty, they should be acquitted than to risk an innocent person being convicted.