“Best-evidence rule” – Written contracts – Oral Evidence – It is in reality declaring a doctrine of the substantive law, namely, in the case of a written contract, that all proceedings and contemporaneous oral expressions of the thing are merged in the writing or displaced by it. (See Thayer’s Preliminary Law on Evidence, p. 397 and p. 398; Phipson’s Evidence, 7th Edn., p. 546; Wigmore’s Evidence, p. 2406.)
Read here: 2003 PLRonline 0007