In logic

Logic

In philosophy, Logic is the formal systematic study of the principles of valid inference and correct reasoning. Logic is used in most intellectual activities, but is studied primarily in the disciplines of philosophy, mathematics, semantics, and computer science...

, 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 the opposite of the analogy

Analogy

Analogy is a cognitive process of transferring information or meaning from a particular subject to another particular subject , and a linguistic expression corresponding to such a process...

. 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.

Examples

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

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

Statutory interpretation

Statutory interpretation is the process by which courts interpret and apply legislation. Some amount of interpretation is always necessary when a case involves a statute. Sometimes the words of a statute have a plain and straightforward meaning. But in many cases, there is some ambiguity or...

 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:

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

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.

Analogy or e contrario?

The problem of whether a yet-unsolved problem should be solved by an analogy or by e contrario is often not solved itself. In law, analogies are often forbidden in penal law

Penal law

In the most general sense, penal is the body of laws that are enforced by the State in its own name and impose penalties for their violation, as opposed to civil law that seeks to redress private wrongs...

 based on the nulla poena sine lege

Nulla poena sine lege

Nulla poena sine lege is a legal principle, requiring that one cannot be punished for doing something that is not prohibited by law. This principle is accepted as just and upheld by the penal codes of constitutional states, including virtually all modern democracies...

principle found in almost all modern constitutional states. On the other hand, e contrario is not usually used in civil law

Civil law (common law)

Civil law, as opposed to criminal law, is the branch of law dealing with disputes between individuals or organizations, in which compensation may be awarded to the victim...

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.