A specifically human act is one that is willed. Breathing, sneezing, scratching an itch, etc., are not specifically human actions - monkeys and kittens breathe, sneeze and scratch as well. Applying to a university, studying for a test, visiting an art museum, calling up a friend, etc., are specifically human acts. A human action is constituted by three elements, and these are 1) the moral object, 2) the motive, and 3) the circumstances.
The moral object of an action answers the question: "What is being done?" or "What are you doing?" For example, I am exercising, or I am telling you something that I know to be false, or I am shooting a gun, or helping an old lady cross the street, etc.
The motive of an act answers the question: "Why is it being done?" or "Why are you doing that?" For example, I am working out so that I can run the Boston Marathon, or I'm lying so that you won't be upset with me, or I am practicing my shot to improve my aim and accuracy, or I am hoping that this old lady will slip me a five-dollar bill for helping her, etc.
The circumstances are the incidental or essential conditions that accompany an event; the when, where, how, and other possibly relevant factors. For example, target practice in an unpopulated rural area; or I am 40 years old, unemployed, and have a family that depends on me; or I am in a court of law and have just taken an oath; or it is winter, the roads are icy, and this old lady is trying to cross a busy intersection, etc.
A more specific moral precept regarding the elements of the human action is the following: If any one of the elements of the human act is evil, the entire act is morally evil or deficient.
The reason is the following: good means fullness of being. Evil, on the other hand, is a privation, a lack of something that should be there. Therefore, in order for a specifically human action to be good, it must be good in its entirety, that is, each element must be good, otherwise it is lacking in a fullness that it ought to possess.
The following are a few simple examples.
A moral evil involves a deficiency in the will. This means that the will is not willing in a way that is consistent with a will towards integral human fulfillment. For example, I certainly will my own fullness of being, but I do not necessarily will your well-being. I must freely choose to will your well-being, and so it is possible for me to choose a course of action that appears to be good for me but is really harmful for you. In doing so, my will is not open to the full spectrum of human goods. It is a deficient will; a good will is one that wills the good, wherever there is an instance of it.
Some actions have two effects simultaneously, a good effect and an evil effect. Such actions are genuine dilemmas. For example, if the police officer shoots, he will free the hostage, but he will also likely kill the perpetrator; if the doctor removes the cancerous uterus, he will save the woman's life, but removing the uterus will also kill the child; if a soldier launches the missile, he will stop the tank, but he will also kill everyone in the tank. What does a person do in such a scenario? Good is to be done, and evil is to be avoided, but in these scenarios, it is not possible to avoid evil when good is to be done.
Double effect is a principle that governs such choices. There are four conditions that must be met before a person may proceed with such an action. The following are the conditions:
1) The action itself (not its effects) must be good or morally indifferent. For example, there is nothing intrinsically evil about removing a cancerous uterus - it is a medical action - , and nothing intrinsically evil about firing a gun.
2) One may not positively will (or intend) the evil effect, only permit it as an undesirable side effect of an otherwise good action. For example, in removing the cancerous uterus, the doctor knows that the child cannot survive, but he does not will or intend the child's death, he only permits or allows it.
3) The good effect must proceed directly from the action itself, and not the evil effect. For example, the mother lives as a result of the removal of a cancerous uterus, not as a result of the death of her child. The hostage is free not as a result of the death of the perpetrator, but as a result of being rendered unable to fire off a shot (he was shot in the occipital region of the head, which prevented him from pulling the trigger on the gun). If the good effect were to proceed from the evil effect, one would have to will the evil effect as a means to the good effect, which violates an important precept of natural law (namely, one must not willingly do evil to achieve good).
4) The good effect must be sufficiently desirable to compensate for the allowing of the evil effect. What this means is that there are circumstances in which we may not permit or allow an evil effect. For example, a young man steals an automobile; the officer has an obligation to try to stop him from getting away. He could shoot the young man as he is driving away from the officer, that is, through the back window. In doing so, he may kill the thief. The good effect in this case is that the officer prevented a car from being taken; the evil effect is the death of the young man. Is the good effect sufficiently desirable to compensate for the allowing of the evil effect? It seems not. The good and evil effects are not proportionate, in this case. Had the young man been driving toward the officer in order to run him over, however, the officer would probably be justified in discharging his firearm in an attempt to stop the car and save his own life.