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Criminal law: General principles

Read more about the different general principles of criminal law, from actus reus to mens rea with our analysis and dedicated essays here.

The Actus Reus

Most crimes need an actus reus to have been committed to make the defendant guilty.

The actus reus is the willed voluntary action of the defendant – it is the doing part of the offence – a positive act. In theft it will be taking someone else’s property. In ABH it is causing a certain level of injury to the victim. An omission, or failure to act, can be criminal but only where the defendant owes some kind of duty to the victim. The duty can because of failure to deal with a dangerous situation, failure to do job properly or failure to look after a member of family.

Some crimes are result crimes so the actus reus will only be committed if a result occurs. In murder or manslaughter there has to be an unlawful killing. To charge section 20 or s18 a serious injury has to be caused.

The Mens Rea

The mens rea is the defendant’s state of mind at the time of committing the actus reus. The defendant’s motive is not relevant but may be taken into account in sentencing. The jury decide if the defendant had the required mens rea.

There are two forms of mens rea - intention and recklessness.

Intention is the defendant’s decision to bring about a prohibited consequence. It can be further divided into direct and indirect intent. Direct intent is said to be where the result is the defendant’s aim or purpose. Indirect intent is where the defendant did not intend the result, but it is said to be a virtual certainty of his actions and he foresaw that to be the case.

Recklessness is a subjective and is where the defendant recognised a risk but still went ahead with the action.

Specific intent crimes require proof of intent only. Basic intent crimes require proof of either intent or recklessness.


Causation is the link between the defendant’s actions and the end result. It is always necessary to prove that the defendant caused the result – the injury or death of the victim. There are three tests to prove causation:

o The ‘but for’ test – but for the defendant’s actions the result (the injury) would not have happened. This is factual causation and is for the jury to decide.

o The substantial test – did the defendant’s actions substantially (more than minimally) cause the end result (the injury). This is legal causation and is for the judge to decide.

o Novus actus – was there an intervening act that removes the defendant’s liability? This is a legal test for the judge to decide. It may have to be considered whether the victim contributed to their injury. You have to take your victim as you find them - if the victim is suffering from a pre-existing injury which the defendant makes worse this test may still be satisfied. If the victim contributes to their injury the defendant may still have caused the result.