Every crime essentially has Four elements: Mens rea, Actus Reus, human being, and injury caused. It is important we understand the first two elements i.e the physical element, and the mental element individually and in relation to one another.
What is Actus Reus?
Actus Reus refers to the physical element of a crime. It is not what you think but what you do. There must be an overt act in relation to the crime. An overt act may be referred to as an act put in action to achieve some intention.
In other words, it is the guilty act that follows the guilty intention.
What is Mens Rea?
Mens Rea refers to the intention behind an ACT. The intention of a person makes them act when this intention is guilty, the same act constitutes a crime provided other conditions are also fulfilled.
In other words, Mens Rea means some blameworthy mental condition irrespective of the fact that such an intention is constituted by knowledge otherwise.
The only time when this mental element is not needed to establish criminal liability is when the statutory provision directly lists the offence as under strict liability. That is, irrespective of the fact that the wrong-doer had no guilty intention, the wrong-does will be criminallty liable.
Examples of such Mens Rea exceptions are Selling of adulterated food, rape and, etc
Difference between Actus Reus and Mens Rea
One major difference between Mens Rea and Actus Reus is that the first one is a physical element that is some overt act in connection with the crime while the later i.e Mens Rea is a mental element behind the criminal act.
A attacked B with a knife in order to occupy his property.
In the above case, the thought of Illegally occupying the property of B is the guilty intention i.e Mens Rea while Attacking B with a knife is the Actus Reus i.e an overt act to reach the intention.
Mens Rea and Actus Reus together
The Latin Maxim, actus reus nisi mens sit rea together means that the act itself does not constitute an offence unless done with a guilty intention.
These two elements must combine together to form a crime unless the act is statutorily declared as triable without criminal intention as discussed above.
Another example would be,
A person with neurological disorders who strikes another during a seizure or attack has not committed the offence of battery because the essential mental element is missing and his act does not concur with his intentions.
This is based on the principle of criminal law that the law does not punish an innocent person.
On the other hand, if the same person who knows he should not drive but does and causes injury, he may be guilty of reckless behavior and criminal liability should follow.