Primary evidence, as distinguished from secondary; original, as distinguished from substitutionary; the best and highest evidence of which the nature of the case is susceptible. A written instrument is itself always regarded as the primary or best possible evidence of its existence and contents; a copy, or the recollection of a witness, would be secondary evidence.
Law Dictionary – Alternative Legal Definition
Means the best evidence of which the nature of the case admits, not the highest or strongest evidence which the nature of the thing to be proved admits of: e.g. a copy of a deed is not the best evidence; the deed itself is better. 2. The rule requiring the best evidence to be produced, is to be understood of the best legal evidence. It is relaxed in some cases, as, e.g. where the words or the act of the opposite party avow the fact to be proved.