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[Latin, The ground or reason of decision.] The legal principle upon which the decision in a specific case is founded.
The ratio decidendi is also known as the rationale for a decision.
West's Encyclopedia of American Law, edition 2. Copyright 2008 The Gale Group, Inc. All rights reserved.
ratio decidendi‘the rule in a decision’. This is a crucial part of the understanding of the way in which the common law works. Once a system has been adopted of binding PRECEDENT, it has to be discovered what it is in the previous decision that binds the court later in time. While it is sometimes possible to peruse the opinion of the judge to find the rule, this is not by any means a reliable way of discovering the rule in the case. The soundest general method is to discover the material facts of the case, determine what the decision was and then to draw the proposition that most closely marries the material facts to the actual decision. It is difficult enough to do this with a single opinion but very much harder with multiple opinions such as come from the Court of Appeal, the Inner House and the House of Lords. Sometimes it is said to be impossible to form a ratio of general application. Anything that is said that is not part of the ratio is said to be an OBITER DICTUM.
Collins Dictionary of Law © W.J. Stewart, 2006