To know properly is to know a thing in its reason, and by its cause. We are truly said to know anything, where we know the true cause thereof.

Explanation

(English-origin maxim.) True understanding of a legal rule requires knowing its reason and cause — cite this to challenge a party who invokes a rule mechanically without understanding the principle that animates it.

Where this applies

• A lawyer mechanically cites the statute of limitations without understanding its underlying purpose; the court rejects the argument because applying a rule without knowing its reason is not true knowledge.
• A student memorizes the holding of a case but cannot explain why the court reached that result; the professor points out that knowing a rule without its cause is not genuine understanding.
• An agency enforces a regulation by rote but cannot articulate the policy it serves; a reviewing court holds the enforcement arbitrary because the agency never grasped the reason behind the rule.