Unless the contrary is specified, every criminal offence requires both a criminal act, expressed in Latin as the actus reus, and a criminal intention, expressed as mens rea. 

Mens rea is often described as the “mental element” in a crime. It can include what used to be known as “malice aforethought”, ie conscious planning or intent, as well as something culpable but less deliberate, such as recklessness or negligence.

A crime which does not require any proof of mens rea to secure a conviction is known as a one of “absolute liability”. One in which one or more elements (but not all) of the actus reus requires no proof of mens rea, or that requires the defence to disprove criminal intent once the prosecution have established an initial case, is known as one of “strict liability”. A statute creating such an offence needs to state expressly (or by necessary implication) that the offence does not require proof of mens rea. Otherwise, such a requirement will be implied into the statute.

A famous case about mens rea is Sweet v Parsley [1970] AC 132 in which the House of Lords had to consider the offence of “being concerned in the management of any premises used for” smoking cannabis, contrary to section 5 of the Dangerous Drugs Act 1965. The defendant was convicted despite having been unaware that her premises were being so used. Her appeal was allowed. The headnote explains that:

“Mens rea is an essential ingredient of every offence unless some reason can be found for holding that it is not necessary, and the court ought not to hold that an offence is an absolute offence unless it appears that that must have been the intention of Parliament.”

In his judgment Lord Diplock cites with approval the analysis of Stephen J (sitting in the Court of Crown Cases Reserved) in the earlier case of R v Tolson  (1889) 23 QBD 168, 187 that 

“The full definition of every crime contains expressly or by implication a proposition as to a state of mind. Therefore, if the mental element of any conduct alleged to be a crime is proved to have been absent in any given case, the crime so defined is not committed; or, again, if a crime if fully defined, nothing amounts to that crime which does not satisfy that definition.”

Lord Diplock goes on to explain, at p 163, that Tolson’s case:

“laid down as a general principle of construction of any enactment, which creates a criminal offence, that, even where the words used to describe the prohibited conduct would not in any other context connote the necessity for any particular mental element, they are nevertheless to be read as subject to the implication that a necessary element in the offence is the absence of a belief, held honestly and upon reasonable grounds, in the existence of facts which, if true, would make the act innocent.”