The rule that the “best evidence must be given on which the nature of the case permits” [FN1] is an old rule has gone by the board long ago. Underpinning that rule was an understanding that documentary proof is usually more reliable than human evidence.
The modern rule only requires that an original (which includes true copies or duplicate originals) should be tendered, if available. When an “original” document is unavailable, exceptionally, secondary evidence is admissible to prove the document. The court must be satisfied that an original document existed, but is unavailable: e.g. has been lost or destroyed, or is in the possession of a third party from whom production cannot be compelled.