A more thorough explanation:

Definition: Ratio decidendi is a Latin term that means "rationale for the decision." It refers to the key factual point or chain of reasoning in a case that drives the final judgment. When courts use earlier cases as precedent, they often ask parties to be very clear about how they interpret the main guiding principle or ratio decidendi of the earlier case.

Example: In the case of R v. Dudley and Stephens, the defendants were charged with murder after killing and eating a cabin boy while stranded at sea. The court found them guilty but reduced their sentence to six months in prison. The ratio decidendi of the case was that necessity is not a defense to murder. This means that even if someone feels they had no other choice but to commit a crime, they can still be held responsible for their actions.

Explanation: The example illustrates how the ratio decidendi is the key principle that guides the court's decision. In this case, the court found that even though the defendants were stranded at sea and felt they had no other choice but to kill and eat the cabin boy, they were still guilty of murder. The ratio decidendi of the case was that necessity is not a defense to murder, which means that even if someone feels they had no other choice but to commit a crime, they can still be held responsible for their actions.