Let every one employ himself in what he knows.

Explanation

(English-origin maxim.) A person should confine himself to his own trade or profession — one who ventures beyond his competence assumes the risk of error and may not claim the protections afforded to a skilled practitioner.

Where this applies

• A plumber who tries to perform his own electrical wiring and causes a house fire cannot claim the same protections as a licensed electrician — he ventured outside his competence.
• A lawyer who gives medical advice to a client, leading to injury, is held to have acted outside his expertise and bears the risk of that error.
• An accountant who drafts a legal contract without a law license cannot claim the work product should be judged by the lenient standards applied to non-professionals — he should have stuck to what he knows.