A party proposes to prove, by a copy, the contents of a document said to be lost. What must be proved…

Section 141(2) says that where evidence is admissible only on proof of some other fact, that fact must be proved first, unless the Court accepts an undertaking to prove it. Illustration (b) to section 141 says the person producing the copy must prove the loss of the original before producing it, so the answer is "That the original is lost, proved by the party producing the copy". The burden is not on the other side, and a photocopy is still secondary evidence.