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A contrario is a Latin term that translates to from the opposite in English. It is a legal maxim used in legal reasoning and arguments to denote an inference or conclusion drawn from known, established, or admitted facts which are contrary or opposite to the case at hand. Essentially, it is used to argue that if a rule or principle applies in one situation, then it should not apply in an opposite or different situation.
For instance, a lawyer might use a contrario when arguing about a law that states that only people over the age of 18 can vote. If a case arises where a 17-year-old is suing for the right to vote, the lawyer might argue a contrario that since the law explicitly states that only individuals over 18 can vote, it implies that those under 18 are not allowed to vote.
In another example, if a law states that landlords must give tenants 30 days’ notice before eviction, and a landlord gives a tenant only 15 days’ notice, a lawyer could argue a contrario that the landlord’s action is unlawful because it directly contradicts the specified rule.
So, the a contrario argument is used to show that if the law has specifically stated one condition, the opposite condition does not apply unless explicitly stated in the law.