A crime is not considered to have been done unless the intention of the accused was to do so.
Easy way to remember
Actus = The act or action
Non facit = not doing / not done
reum = the accused / defendent
nisi = unless
mens = mental condition / of the mind / intention
sit = to be
rea = reum / the accused / defendent
This maxim holds good for criminal law, where not only the act of the accused must be proved, but the intention of the accused to do that specific act must also be proven to show guilt and sentence him for that particular crime.
The act follows the intention.
By this principal, the accused must have the intention to perform a certain crime, which caused him to act in that way. The mental state must be such that the accused had the intention to commit that act and the knowledge of its outcome.
Only when the act committed and the state of mind of the accused is in concurrence, can he be sentenced for committing a crime.
No where in this maxim is the word guilt mentioned or implied. Hence, this maxim does not talk about guilt, which needs to be proven later, it only talks about the mental state of the accused which maybe further extended to the intention, knowledge or foresight of the accused.
The interpretation that this maxim means “an act does not make a person guilty unless (their) mind is also guilty” is not completely correct.
An accused who commits a motor vehicle accident which leads to death of the victim, may be charged for murder, if he had the intention of killing the accused i.e. his act and mind worked in unison to execute the crime. Else, it would be considered an accident or negligence attracting a lesser punishment.