Reason is the formal cause of custom.

Explanation

A custom that lacks a rational foundation is no true custom — when reason departs, the custom falls with it.

Where this applies

• Someone argues that a custom should be followed even though its original reason no longer exists.
• A practice continues out of habit but has no rational basis anymore.

See also

• 1.3.4 A custom, however great its authority, should never prejudice manifest truth.
• 1.5.20 Reason is the soul of law; when the reason of the law changes, the law itself changes.