The best evidence rule is the legal requirement for an original copy of any document to be used within a court case if it is available. The only time a copy of a document is admissible is when the original cannot be obtained. This law originated in Britain in the 18th century when forgery was rife and not as easily noticeable.
The idea behind this is that the only way to prove something is absolute is through viewing the original documetation as such would contain either watermarks or other markings that would ensure that the document existed validly.