What is the test for establishing that a written agreement is binding and binding?

In Ross v. Williamson (at p. 186): "… [W]hen a party endeavours to prove by oral testimony the contents of a written document, the court, before giving effect to such testimony should be convinced that all the terms have been proven. I do not think it sufficient for the party undertaking such a duty to furnish evidence of certain clauses which support his claim. He must do more; he must set out the whole instrument so that the court may be able to give effect to all its provisions, and that by testimony of the clearest nature. I do not mean that the document must be set forth in evidence - in its very words - but the exact sense and effect of the document must be shewn. "That this, in most cases, will be a difficult, indeed an almost impossible task, is apparent; but such burden must be carried by him who asserts a written agreement, which he says has been destroyed, and the existence of which is denied by his opponent." (my emphasis)