Argumentum e contrario

In logic, an argumentum e 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.

Legal examples

"§ 123 of the X-Law says that green cars need to have blue tires. Therefore, red cars don't have to have blue tires."

On the other hand, this example:

"§ 456 of the Y-Law says that it's irrelevant whether a message is sent by letter or by telegraph. Therefore, messages cannot be sent by fax machines."

Novel legal cases often hinge on more cogent arguments of the form:

"§§ 455–457 of the Y-Law specifically cover messages sent by letter or by telegraph, and do not mention fax machines or electronic mail, and thus cannot be held to apply to either of the latter."