What is the test for determining the validity of a contract?

'Where any doubt exists as to the purport of the [206 Cal.App.2d 291] parties' dealings as expressed in the wording of their contract, the court may look to the circumstances surrounding its execution--including the object, nature and subject matter of the agreement [citing case]--as well as to subsequent acts or declarations of the parties 'shedding light upon the question of their mutual intention at the time of contracting.' [Citing case.]' (Barham v. Barham, 33 Cal.2d 416, 423, 202 P.2d 289, 293.)