An excuse is a claim that aims to shield an agent from condemnation for what appears on first glance to be a wrong act. A good excuse successfully shields the agent, while a poor excuse fails to do so.
If we accept the central claim of desirism - that desires are the primary object of moral evaluation - we can see excuses as claims that attempt to block the inference from what appears to be an act that a person with good desires would not have performed to the desires of the agent. In short, an excuse claims, "It may appear to be the case that a person with good desires would not have performed such an act. However, a more careful look at the facts of the matter shows that this is not the case."
Accident: A car goes through an intersection and hits a pedestrian in a crosswalk. This creates a state of affairs that a person with good desires would have sought to avoid. The agent in this case claims it was an accident - the brakes failed. That is to say, "I could not have stopped the car, even if I had wanted to." In fact, an examination of the car shows that the brakes had been tampered with. The agent is excused.
False Belief: A traveller at an airport grabs a suitcase and starts walking off. Another person stops her and says, "That is my suitcase." In fact, an examination shows that she is right. The first passenger finds her own bag - which is nearly identical to the one she took. A person with good desires does not walk off with the property of another. However, if an act can be explained in terms of a mistaken belief, then this blocks the inference to bad desires. Please note, however, that a proper awareness if the harms of false belief would motivate a good person to double-check his facts. Recklessly formed belief betrays a lack of concern in avoiding these types of harms.
Denying Harm: A kid is discovered spraying graffiti on his neighbor's car. This is something that a person with good desires would not do. However, when accosted, the kid reveals that the spray paint comes off with soap and water. He points to the bucket of soapy water and announces that after playing a trick on his neighbor, he intends to was his neighbor's car. No harm done. The act us excused. Technically, though, "harm" is too broad a word. More accurately, this excuse involves denying that one is creating a state of affairs that a person with good desires would avoid creating. It often involves harm, but not always.
Consent (as an excuse): A man violently slap and shoves a woman. This is something a person with good desires would not do. Somebody walking by who witnesses such an act would, at first glance say that the man did something wrong. However, it turns out that the two are actors. The woman has agreed to let the man slap her (not too hard) in the course of rehearsing or performing a scene. She has given her consent. The man is excused. Even a person with good desires can slap a woman under these circumstances.
Greater Good (as an excuse): A person arrives late for an appointment. A person with good desires has an aversion to making others wait. Eventually, the tardy person calls those who are waiting. She reports that the car in front of her as she drove to the meeting struck and seriously injured a kid on a bicycle. Being a doctor, she stopped and gave aid to the injured child. A good person would, indeed, have an aversion to making others wait, but her desire to help an injured child would be stronger. Her lateness is excused.
Deserved Punishment (as an excuse): A man takes another man and pushes him into a small room, closes, and locks the door. This is something a person with good desires would not do to another person. However, in this case, we discover that the person confined to the room has a history of violent assaults against others. He has been convicted of these crimes in a court of law where he had been given a fair opportunity to defend himself and explain his actions. The agent who locks the person in his cell makes the claim that the criminal deserves to be punished. He offers this as his excuse.
What all excuses have in common is they take an action from which it appears to follow that a person with good desires would have done something differently and says, "In fact, a person with good desires would not have done something differently."
A person with good desires would have an aversion to running over pedestrians in a sidewalk, taking somebody else's suitcase, spray-painting graffiti on a neighbor's car, slapping a person, being late for an appointment, or confining a person to a small room against his will. However, there are events that a person cannot prevent no matter how good his desires are. Actions are based not only on desires but beliefs, and false beliefs generate what appear to be wrong actions. The agent may have taken steps to avoid the harms that would activate the aversion in a person with good desires, or obtained the consent of those who would be harmed. The person with good desires may have been forced by a stronger aversion (to letting a child suffer or die) to set aside a weaker aversion (being late for an appointment). The agent may be acting within an institution of reward and punishment.
Excuses exist as a part of our moral life because the ultimate claim in morality is, "A person with good desires would not have done what you did." There are cases in which this appears at first glance to be true, where a closer examination of the facts reveals that it is not true. Claims that attempt to bring forth these additional facts are called "excuses". A good excuse actually does block the inference. A poor excuse claims to block the inference but does not succeed.
There is much more to be said about each of these six types of excuse. Each will be covered separately in much greater detail.