Explanation:

- Professional opinions: These are views from esteemed legal experts or scholars. They are considered influential but not binding. They help guide interpretation and understanding of law.

- Principles of morality: These refer to fundamental values and ethical standards embraced by society. They can shape legal thinking and influence judicial decisions.

- Principles of equity: Equity complements strict legal rules by focusing on fairness and justice. It fills gaps and resolves conflicts where rigid laws might fall short.

- All of the above: As Keeton suggests, all these elements can be persuasive in shaping and guiding law. They collectively enrich legal interpretation and decision-making.