You ought to know with whom you deal.

Explanation

(English-origin maxim.) Every party to a transaction bears the duty of ascertaining the identity, capacity, and authority of the person with whom he deals — ignorance of a counterparty's lack of authority or title is no defense when reasonable inquiry would have revealed it.

Where this applies

• A person enters a contract without verifying the other party's authority and later suffers loss.
• Someone relies on an imposter's representations without checking who they really are.