Argumentum e contrario

In logic, an argumentum e contrario (Latin: 'argument from the contrary'; also a contrario or ex contrario), also known as appeal from the contrary, denotes any proposition that is argued to be correct because it is not disproven by a certain case. It is the opposite of the analogy. When analogy is allowed, e contrario is forbidden and vice versa. Arguments e contrario are often used in the legal system as a way to solve problems not currently covered by a certain system of laws. Although it might be used as a logical fallacy, arguments e contrario are not by definition fallacies.

In law, the use of the argumentum e contrario finds its footing in the Latin maxim: ubicumque lex voluit dixit, ubi tacuit noluit that runs as follows: If the Legislator wished to say something, he would do that expressly.