What is the test for determining the meaning of a clause in a contract?

In Hill v. Nova Scotia (Attorney General), 1997 CanLII 401 (SCC), [1997] 1 S.C.R. 69, Cory J. cited this principle with approval and noted, at para. 20: What the statement quoted means is that in determining what was contemplated by the parties, the words used in a document need not be looked at in a vacuum. The specific context in which a document was executed may well assist in understanding the words used. It is perfectly proper, and indeed may be necessary, to look at the surrounding circumstances in order to ascertain what the parties were really contracting about. [Emphasis added by Cory J.]