Mens rea is a term typically used in criminal law to refer to the mental elements that one must possess to be guilty of a crime.
It is also used when making moral judgments to refer to the mental elements that make an agent actually responsible for (culpable of) a moral transgression.
For example, let us assume that a person hooks up a car so that when somebody tries to start the car it sets off a bomb in a nearby building. The car owner, unaware of this tampering, gets in the car and tries to start it. The bomb goes off, killing a group of innocent bystanders. Is the person who starts the car guilty of murder?
In both the criminal and the moral case, the standard answer is that she is not guilty. Though she turned the key that detonated the bomb that killed the people, her connection with the outcome does not contain the mental elements to establish mens rea or moral culpability.
If a store shelf gets bumped and an item falls unknowingly into bag that a customer is carrying and the customer leaves the store with the item he is not a thief. If the brakes on a car fail and the driver crashes through a fence, the driver is not a vandal.
Furthermore, in morality as in law, we recognize a distinction between the murderer who aimed a gun known to be loaded at a person, flips off the safety, and squeezes the trigger - as opposed to the agent who, while handling a gun, accidentally shoots somebody nearby. We recognize the difference between premeditated murder and negligent homicide.
These illustrate just a few ways in which mental elements play a significant role in judgments of criminal and moral culpability.
Four Categories of Culpability
Typically, law recognizes four categories of culpability.
1. Intentional/Purposeful. A person’s act is intentional or purposeful if the agent actually sought to realize the end in question as an end or as a means to an end. If the assassin aims a gun and pulls the trigger seeking to kill a person, then this is intentional homicide.
2. Knowing. A person knowingly acts to realize a state of affairs if he is aware of the fact that the act will realize a state of affairs, though does not specifically aim to bring it about. For example, a demolition company demolishes a house knowing that there are people inside. They did not plan to kill anybody - but they did not let the fact that their act would kill people get in the way of blowing up the building.
3. Reckless. An action is reckless when the agent knows that there is a risk of others being harmed. A person who fires a gun randomly down the street might not harm anybody. However, he not only creates a risk but does so with the knowledge that somebody might get hurt. This qualifies his actions as reckless.
4. Negligent. An action is negligent when a person ought to have known that there was a risk of harm. A drunk driver may believe that he can get home safely. However, the fact that fails to respect the risk that he is creating for others does not absolve him of guilt. A concerned individual would not make such a dangerous assessment of their own abilities. They would have known and respected the fact that drunk driving creates a risk and acted accordingly. The drunk driver in this case is negligent.
Morality recognizes these same distinctions. In fact, the recognition of these states in the law follows the moral distinction. They are included in the law in order to capture a particular part component of our moral judgments.
The Source of Culpability
To see what culpability aims at, let us look at a thought experiment:
Let us assume that Person A creates a device that allows him to control Person B’s body. While Person A has this control, he has Person B take a weapon, go to Person C's house, aim the weapon at Person C and fire.
Let us ask, who is guilty of this murder? Is it Person A, the one whose mental elements motivated the murder? Or is it Person B - the person who aimed the gun and pulled the trigger? Or do they share responsibility?
The standard answer is that A is guilty. B is innocent - he was a tool, just like the gun and the bullet were tools. A gets 100% of the responsibility, while B gets none of it.
That is to say, the culpability applies entirely to the agent who provided the mental states. He is the one with the required mens rea (or guilty mind) essential for establishing culpability of blameworthiness.
Of the various mental states, what does culpability aim at?
The options here are to ask whether the elements of a "guilty mind" are to be found in an agent's beliefs, or in his desires.
If a person walks into a open garage and walks out with a set of power tools, there appears to be no "culpable belief" that makes the agent blameworthy.
Beliefs are certainly relevant to culpability. If the agent believed that the garage and the power tools were his, he can offer an excuse of false belief to save himself from condemnation and punishment for taking the tools.
However, the belief is relevant because of what it implies about the agent's desires. A person is not expected to have an aversion to walking into his own garage and using his own tools. However, he is expected to have an aversion to taking the property of another without consent. The person who takes tools from a garage that he could not possibly believe is his own with no reason to believe that the owner gave consent demonstrates that he lacks this aversion that people generally have many and strong reasons to promote. Once the material facts establish that the agent likely lacks the requisite desires, the mental element of a "guilty mind" is established and the conclusion that the agent is blameworthy has been established.
This, then, is where we find mens rea or the "guilty mind" - in the desires that an agent lacks that people generally have reason to promote, or in the desires an agent has that people generally have reason to inhibit.
Desirism, then, provides us with a way of explaining the concept of culpability as it is used in moral practices. Desirism holds that the primary object of moral evaluations are malleable desires. We see in the element of culpability or "mens rea" that moral blameworthiness does appear to attach itself to malleable desires precisely because they are the desires that people can and have reason to mold using social forces.