Restatement (Second) of Contracts § 21 (Am. Law Inst. 1981).
Restatement (Third) Torts: Products Liability § 1 (Am. Law Inst. 1997).
The Restatements of the Law are valuable secondary sources that aim to distill a concise set of principles or rules from the common law. In general, these rules "restate" what the law is, not what it should be. However, on occasion, the Restatements do provide recommendations on a rule of law.
The American Law Institute (ALI) is a group of prominent judges, lawyers, and professors, that publishes the Restatements. The drafting process of the Restatements is painstaking and can take anywhere from 9-21 years. Various ALI committees examine cases and identify trends in the common law and then translate their findings into legal principles or rules. Several versions of the Restatements are drafted before a final version is adopted. Restatements are not primary law. Rather, they are legal commentary. However, due to the prestige of the ALI and the careful drafting process, many courts consider them highly persuasive authority.
The Restatements cover broad areas of law, such as Torts or Agency. Not every legal subject is covered by the Restatements. You can access the complete list of the subjects covered in current and past Restatements by clicking on the link "ALI Restatement Checklist" on the lefthand side of the screen. Each volume of the Restatements is organized into chapters. Each chapter is then organized into titles, and each title is divided into sections. Each section contains a concisely stated rule of law, comments and illustrations which are used to clarify the rule, explanations of the rule's purpose, and major exceptions to the rule.