The concept of mens rea requires establishing a defendant's state of mind—whether they intended the consequence of their actions or foresaw the risk of it happening. This is often described as a subjective test, which assumes that a person’s mental state can be ascertained, though it is scientifically impossible to directly measure someone's intention, as noted in Hyam by Ackner J. Without direct evidence, like a confession, courts must infer mens rea from circumstantial evidence, such as the defendant’s conduct, statements, or motive.
Historically, courts have used the maxim that a person is presumed to intend the natural and probable consequences of their actions, which tends to shift the assessment from a subjective to an objective test of mens rea. This was rigidly applied in Director of Public Prosecutions v Smith (1961), where the House of Lords concluded that the test for mens rea in murder was whether a reasonable person would have foreseen harm, regardless of what the defendant actually intended. This essentially transformed murder into a crime based on negligence, leading to widespread criticism.
The Criminal Justice Act 1967 s.8 reversed this presumption, stating that intention and foresight must be assessed subjectively by considering all evidence, not just whether a result was a natural consequence. While this clarified the theoretical approach, the practical challenge of proving mens rea remains, as juries often still rely on drawing inferences from the defendant's behavior, applying their own standards of what a reasonable person would have foreseen.
The core mens rea concepts, such as intention, recklessness, and negligence, vary depending on the offense and can be defined by statute or case law. For instance, theft requires proof of dishonesty, while recklessness suffices for assault.