Generally includes sources that summarize, explain, or analyze the law, including treatises, law review articles, and restatements. Although not authoritative, these sources are valuable research tools. In addition, when no primary authority governs a dispute in a jurisdiction, these sources may provide guidance to a court in fashioning a new rule.
Treatises and articles frequently contain policy discussions that analyze novel issues and the social consequences of legal rules. If a case involves an issue of broad consequence requiring a policy review, these sources may be helpful in developing and supporting policy arguments.
Although you need to be familiar with the policy views of the commentators, use these opinions sparingly in argument. Primary authorities are the best basis for policy arguments. It is one thing to extract a policy discussion from a case and another to advance a social theory that merely represents an individual's opinion. When a court relies on policy to support a legal holding, it has elevated the policy from social to legal status. The policy argument also should have special strength because it survived scrutiny in the adversarial process.
If you need to argue policy, find support for your contentions. Thus, if the cases do not support your arguments, you may rely on secondary authority. Try to find a secondary source that is recognized as authoritative or has some special status in the relevant field.
~excerpt from "Persuasive Written and Oral Advocacy"