The document discusses the complexities of determining the proper law of contracts within private international law, highlighting the challenges of ascertaining the applicable law given various connecting factors such as the domicile of parties and place of performance. It outlines two primary theories for identifying the proper law: the Theory of Intention, which focuses on the parties' expressed or presumed intent, and the Theory of Localization, which considers the law of the location most closely connected to the contract. Additionally, the document illustrates these theories through case law and discusses factors like capacity and formal validity that may also influence the choice of law.