Common opinion is good authority in law.

Explanation

(English-origin maxim.) The settled opinion of learned and experienced persons on a point of law or practice carries authoritative weight and may be cited as a legitimate source of legal reasoning.

Where this applies

• A lawyer cites a legal principle that has been recognized by multiple generations of judges and scholars — the court gives it weight because long-settled opinion among learned authorities is itself a valid source of law.
• A party challenges a rule of practice that has been followed without objection for a century — the court upholds it because common opinion over time becomes good authority.
• A novel question arises, and counsel cites the unanimous view of the leading treatise writers on the subject — the court treats that consensus as persuasive authority even without a directly controlling case.