He who does not prohibit when he is able to prohibit, is considered as committing the thing.
Explanation & Origin
Origin – “Qui non prohibit quod prohibere potest assentire videtur” is a Common Law maxim of Latin origin.
Explanation – It is a Common law maxim meaning that one, who has the lawful authority to prevent an act or wrongdoing by another, will be legally bound by the act or liable for the wrongdoing if he fails to prevent it. This maxim is generally applied in cases where a wrongdoing is done by a subordinate or an agent; then the higher authority or the principle can be held liable for the act, if he/she was in the position to forbid the act, as if he/she must have done the wrongdoing. Thus, whoever does not prohibit that which he can prohibit is considered as assenting.
Another alternative for the maxim is “Qui potest et debet vetare et non vetat iubet”, which means whoever is able and should forbid and does not forbid commands.
If there is a contract between the landlord and the tenant that subletting is prohoibitted but the tenant sublets his apartment to a third person and the landlord not only has the knowledge of this but also stands by it, then the landlord will be bound by it for “Qui non prohibit quod prohibere potest assentire videtur”.
In the case of Teasdale v. Teasdale, it was stated that one who enables another to commit a fraud is answerable. A person who has a title to property offered for sale at an auction, and knowing his title to the property, stands by and encourages the sale or does not forbid it, will be bound by the sale, for “Qui non prohibit quod prohibere potest assentire videtur”.
Edited by Vigneshwar Ramasubramania
Approved & Published – Sakshi Raje
 Sel. Ch. Cas. 59.