## Facts

- Thornton was employed as a bus conductor. In the ordinary course of his duties he stood at the rear of the vehicle and gave hand signals to the driver, Mitchell, indicating when it was appropriate to manoeuvre.
- On the day in question a pedestrian was behind the bus. Thornton, failing to keep an adequate lookout, nevertheless gave Mitchell the “all clear” to reverse.
- Trusting Thornton’s signal, Mitchell reversed the vehicle. The pedestrian was struck and suffered fatal injuries.
- Criminal proceedings followed. Mitchell was indicted for manslaughter on the basis that his driving was criminally negligent. The jury, however, acquitted him, concluding that he had exercised reasonable care and that any fault lay in Thornton’s signal rather than in the driving itself.
- Thornton was then tried as a secondary party—specifically, as one who had aided and abetted the alleged manslaughter committed by Mitchell. The trial court convicted Thornton, reasoning that his careless signal had materially contributed to the pedestrian’s death.

## Issues

1. Can a person be convicted as an aider or abettor when the alleged principal has been acquitted because the requisite intent or negligence was not proved?
2. Does secondary liability arise only where the principal’s conduct amounts to a crime, or can it arise independently from the aider’s own conduct?

## Decision

- The Court of Criminal Appeal quashed Thornton’s conviction.
- Their Lordships held that secondary liability is parasitic on the existence of a completed offence by the principal. Since Mitchell’s acquittal rested on a finding that no crime had been committed—because criminal negligence was not established—there was no offence to which Thornton’s assistance could attach.
- The court distinguished cases in which a principal is technically guilty but escapes conviction through personal incapacity (for example, infancy or insanity). In such contexts the conduct itself is still criminal, so liability may extend to an accessory. In contrast, where the acquittal reflects the absence of the actus reus or mens rea altogether, the very basis of secondary liability is absent.

## Legal Principles

- Dependence of secondary liability: The aider’s liability is contingent on proof, to the criminal standard, that the principal’s conduct constitutes an offence. Without that predicate offence, the accessory cannot be guilty.
- No conversion of lawful into unlawful: Assistance or encouragement by a secondary party cannot turn what is, in law, a lawful act into an unlawful act. If the principal’s driving was lawful, Thornton’s negligent signal could not alter its character.
- Distinction between excusable principals and unproved offences:
  - Where a principal is excused because of a personal defence (e.g., insanity), the act in question remains criminal, so an accessory may still be liable.
  - Where a principal is acquitted because the prosecution fails to establish negligence or intent, the act is not criminal at all; accessory liability cannot be superimposed upon a non-existent crime.
- Confirmation of orthodox doctrine: The judgment reiterates the common-law rule that accessories and principals are tried as parties to the “same offence”. If there is no offence, neither party can be convicted.
- Evidential implications: The prosecution cannot rely on proof of an accessory’s assistance alone; it must also establish the principal’s guilt beyond reasonable doubt.

## Conclusion

R v Thornton and Mitchell affirms that secondary liability is derivative. An aider or abettor can be convicted only if the principal offender is shown to have committed a crime. Mitchell’s acquittal for lack of criminal negligence meant that no manslaughter existed in law. Consequently, Thornton’s conviction could not stand. The case remains an authoritative statement that the guilt of an accessory depends on the proven guilt of the principal, subject only to the limited category of personal excuses that leave the crime itself intact.


R v Thornton and Mitchell [1940] 1 All ER 339