Whiteacre

Action of ejectment to recover possession of Whiteacre. P claims to be the owner entitled to possession. D admits P's possession but denies P's ownership. At trial, P proposes to testify that D executed a deed by which he conveyed title in fee simple absolute to P. D objects on best evidence grounds.

What ruling and why? What is so special about writings that there exists a special rule of preference for them?