A custom introduced against reason ought rather to be called a usurpation than a custom.

Explanation

(English-origin maxim.) Custom derives its authority from reason — when a custom contradicts reason, it is not true custom but mere usurpation, and carries no legal weight.

Where this applies

• A government agency claims that because it has always operated a certain way, the practice must be lawful.
• An official defends an overreach as long-standing custom, even though it violates a fundamental right.