1. The principle or rule of law on which a court’s decision is founded (many poorly written judicial opinions do not contain a clearly ascertainable ratio decidendi).
2. The rule of law on which a later court thinks that a previous court founded its decision; a general rule without which a case must have been decided otherwise (this opinion recognizes the Supreme Court’s ratio decidendi in the school desegregation cases). — Often shortened to ratio. Pl. rationes decidendi (ray-shee-oh-neez des-[schwa]-den-dI). Cf. OBITER DICTUM; HOLDING(1).
“The phrase ‘the ratio decidendi of a case’ is slightly ambiguous. It may mean either (1) the rule that the judge who decided the case intended to lay down and apply to the facts, or (2) the rule that a later court concedes him to have had the power to lay down.” Glanville Williams, Learning the Law 75 (11th ed. 1982).
“There are … two steps involved in the ascertainment of the ratio decidendi…. First, it is necessary to determine all the facts of the case as seen by the judge; secondly, it is necessary to discover which of those facts were treated as material by the judge.” Rupert Cross & J.W. Harris, Precedent in English Law 65–66 (4th ed. 1991).
