The two basic elements of a crime are the act of doing that which is criminal, and the intention to carry it out. In Latin this is called the actus reus and the mens rea. In many crimes however, there is no necessity of showing criminal intention, which is why the term "strict liability" is used.
Actus reus: Causation in English law
An English court room in 1886, with Lord Chief Justice Coleridge presidingActus reus is Latin for "guilty act" and is the physical element of committing a crime. It is usually the application or threat of unlawful force, though exceptionally an omission or failure to act can result in liability. Simple examples might be A hitting B with a stick, or X pushing Y down a water well. These are guilty acts and the unlawful application or force. Alternatively, one may have a pre-existing duty to another person and by deliberately not performing it, one commits a crime. For instance, not giving food is an omission rather than an act, but as a parent one has a duty to feed one's children. Pre-existing duties can arise also through contract, a voluntary undertaking, a blood relation with who one lives and occasionally through one's official position. As the 19th century English judge, Lord Coleridge CJ wrote,
“It would not be correct to say that every moral obligation involves a legal duty; but every legal duty is founded on a moral obligation.”
Furthermore, one can become bound by a duty to take reasonable steps to correct a dangerous situation that one creates. In R v Miller a squatter flicked away a still lit cigarette, which landed on a mattress. He failed to take action, and after the building had burned down, he was convicted of arson. He failed to correct the dangerous situation he created, as he was duty bound to do. In many countries in Europe and North America, Good Samaritan laws also exist, which criminalize failure to help someone in distress (e.g. a drowning child). On the other hand, it was held in the U.K. that switching off the life support of someone in a persistent vegetative state is an omission to act and not criminal. Since discontinuation of power is not a voluntary act, not grossly negligent, and is in the patient's best interests, no crime takes place.
If someone's act is to have any consequence legally, it must have in some way caused a victim harm. The legal definition of "causation" is that "but for" the defendant's conduct, the victim would not have been harmed. If more than one cause for harm exists (e.g. harm comes at the hands of more than one culprit) the rule states that to be responsible, one's actions must have "more than a slight or trifling link" to the harm. Another important rule of causation is that one must "take his victim as he finds him." For instance, if P gives his friend Q a playful slap on the head, but Q suffers from a rare cranial condition and dies, then P can be guilty of manslaughter regardless of how unlucky he is to have bickered with Q. This is known as the thin skull rule.
Between the defendant's acts and the victim's harm, the chain of causation must be unbroken. It could be broken by the intervening act (novus actus interveniens) of a third party, the victim's own conduct, or another unpredictable event. A mistake in medical treatment usually will not break the chain, unless the mistakes are in themselves "so potent in causing death." For instance, if emergency medics dropped a stab victim on the way to the hospital and performed the wrong resuscitation, the attacker would not be absolved of the crime.
The interplay between causation and criminal responsibility is notoriously difficult, and many outcomes are criticized for their harshness to the unwitting defendant and sidestepping of hospitals' or the victim's own liability. In R v Dear, a stab victim reopened his wounds while in the hospital and died. But despite this suicidal behavior, the attacker was still held fully responsible for murder.
Mens reaMain article: Intention in English law
Mens rea is another Latin phrase, meaning "guilty mind". It is the mental element of committing a crime and establishes the element of intent. Together with an actus reus, mens rea forms the bedrock of criminal law, although strict liability offenses have encroached on this notion. A guilty mind means intending to do that which harms someone. Intention under criminal law is separate from a person's motive. R v Mohan  2 All ER 193, intention defined as "a decision to bring about... [the actus reus] no matter whether the accused desired that consequence of his act or not." In the special case of murder, the defendant must have appreciated (i.e. consciously recognized) that either death or serious bodily harm would be the result of his actions. In R v Woolin, a man in a fit of temper threw his three month old son onto a wall, causing head injuries from which he died. Although death was certain and the father should have realized, he did not in the least desire that his son be killed or harmed. The English House of Lords sentenced him for manslaughter, but not murder. If a defendant has foresight of death or serious injury the jury may, but is not bound, to find the requisite mens rea.
A lower threshold of mens rea is satisfied when a defendant recognizes that some act is dangerous but decides to commit it anyway. This is recklessness. For instance if C tears a gas meter from a wall to get the money inside, and knows this will let flammable gas escape into a neighbor's house, he could be liable for poisoning. This is called "subjective recklessness," though in some jurisdictions "objective recklessness" qualifies as the requisite criminal intent, so that if someone ought to have recognized a risk and nevertheless proceeded, he may be held criminally liable. A novel aspect of the law on intention is that if one intends to harm somebody, it matters not who is actually harmed through the defendant's actions. The doctrine of transferred malice means, for instance, that if a man strikes another with his belt, but the belt bounces off and hits a nearby woman, the man is guilty of battery toward her. Malice can also be general, so that terrorists who plant bombs to kill random people are certainly guilty.
The final requirement states that both an actus reus and a mens rea coincide. For instance, in R v Church, For instance, Mr. Church had a fight with a woman which rendered her unconscious. He attempted to revive her, but gave up, believing her to be dead. He threw her, still alive, in a nearby river, where she drowned. The court held that Mr. Church was not guilty of murder (because he did not ever desire to kill her), but was guilty of manslaughter. The "chain of events", his act of throwing her into the water and his desire to hit her, coincided. In this manner, it does not matter when a guilty mind and act coincide, as long as at some point they do.
Not all crimes have a mens rea requirement, or the threshold of culpability required may be reduced. For example, it might be sufficient to show that a defendant acted negligently, rather than intentionally or recklessly. In offences of absolute liability, other than the prohibited act, it may not be necessary to show anything at all, even if the defendant would not normally be perceived to be at fault.
England and Wales has strict liability offences, which criminalize behavior without the need to show moral wrongdoing. Most strict liability offences are created by statute, and often they are the result of ambiguous drafting. They are usually regulatory in nature, where the result of breach could have particularly harmful results. An example is drunk driving.