What is the test for renouncing the evidence required to prove the purpose of the contract?

In Reardon Smith v. Hansen-Tangen, [1976] 3 All E.R. 570, Lord Wilberforce stated at p. 5: But it does not follow that, renouncing this evidence, one must be confined within the four corners of the document. No contracts are made in a vacuum: there is always a setting in which they have to be placed. The nature of what is legitimate to have regard to is usually described as 'the surrounding circumstances' but this phrase is imprecise: it can be illustrated but hardly defined. In a commercial contract it is certainly right that the court should know the commercial purpose of the contract and this in turn presuppose knowledge of the genesis of the transaction, the background, the context, the market in which the parties are operating.