Is a contract enforceable if the terms of the contract are not disclosed in writing?

Although 'the intention of the parties is to be ascertained from the writing alone, if possible' (id., 1639), '[a] contract may be explained by reference to the circumstances under which it was made, and the matter to which it relates' (id., 1647). 'However broad may be the terms of a contract, it extends only to those things . . . which it appears that the parties intended to contract.' (Id., 1648.)" (Hess v. Ford Motor Company (2002) 27 Cal.4th 516, 524-525 (Hess).)