In logic, an argumentum e contrario (Latin: "appeal from the contrary" or "argument based on the contrary") denotes any proposition that is argued to be correct because it is not proven by a certain case. It is the opposite of the analogy. 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.
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Here the argument is based on the fact that red cars are not green cars and as such § 123 of the X-Law cannot be applied to them. This requires the law to be interpreted to determine which solution would have been desired if the lawmaker had considered red cars. In this case it's probably safe to assume that they only wanted to regulate green cars and not regulate cars of other colors.
On the other hand, this example:
As with the example above, the argument is based on the fact that the law does not mention fax machines and they must therefore not be used. Here the interpretation that the lawmaker consciously did not mention fax machines is less valid than the assumption that fax machines did not exist at this time and that, were the law passed today, they would have been mentioned. Here the argument e contrario is used fallaciously since it places the letter of the law above its intent.
The problem of whether a yet-unsolved problem should be solved by an analogy or by e contrario is often not solved itself.[1] In law, analogies are often forbidden in penal law based on the nulla poena sine lege principle found in almost all modern constitutional states. On the other hand, e contrario is not usually used in civil law when analogies are possible and desirable. Often, when different solutions are possible for a certain problem, one of them will be an analogy to an existing solution with the other an argument e contrario.