What is the test for establishing a written contract?

In Goss v. Nugent, 110 E.R. 713, Denman, C.J., states at p. 716: "By the general rules of the common law, if there be a contract which has been reduced into writing, verbal evidence is not allowed to be given of what passed between the parties, either before the written instrument was made, or during the time that it was in a state of preparation, so as to add to or subtract from, or in any manner to vary or qualify the written contract; but after the agreement has been reduced into writing, it is competent to the parties, at any time before breach of it, by a new contract not in writing, either altogether to waive, dissolve, or annual the former agreements, or in any manner to add to, or subtract from, vary or qualify the terms of it, and thus to make a new contract; which is to be proved, partly by the written agreement, and partly by the subsequent verbal terms engrafted upon what will be thus left of the written agreement." (Emphasis mine)