The autonomy principle says that you shouldn’t interfere with freely chosen, ethical actions of other people without informed or implied consent.
This means, for example, that you shouldn’t kill others. Serious injury, enslavement, and coercion violate autonomy when they interfere with freely chosen, ethical actions. Respect for autonomy can be more precisely defined by the following principles:
Principle of joint autonomy. You should not do anything that you are rationally constrained to believe is jointly incompatible with the freely chosen, ethical actions of a group of one or more other persons. Throwing a bomb into a crowded street violates autonomy, even if there is no one person you know will be harmed by the bomb, because you know that someone on the street will be harmed.
Principle of interference. Interfering with unethical behavior does not violate autonomy, because unethical behavior is not freely chosen action. It has no coherent rationale and therefore cannot be distinguished from behavior that is not deliberately chosen. For example, you can grab a mugger to prevent him from attacking a victim. However, the interference must not go so far as to prevent ethical action, unless there is implied consent (see next point). For example, you can’t lock someone in a closet to prevent him or her from cheating on income taxes, because this interferes with many ethical actions. Also, the principle refers to interference by an individual, not necessarily by the state. Further argument is necessary to determine when and whether incarceration is justified, for example.
Principle of implied consent. You can interfere with a person’s ethical action, without violating autonomy, when that person implicitly consents to a policy that is consistent with the interference. You can tackle a distracted pedestrian who is about to walk into oncoming traffic, because the pedestrian already has a policy of not walking into traffic and therefore implicitly consents. You can incapacitate a mugger, when necessary, to protect yourself from attack, even though this interferes with ethical as well as unethical actions, because the mugger already has a policy of self-defense and therefore gives implied consent.
Principle of informed consent. You can expose a person or group of people to harm, without violating joint autonomy, when there is informed consent. You can administer a vaccine to 1000 people even if you know that it will make someone ill, if all 1000 give informed consent.
The autonomy principle doesn’t require you to give people anything they want; it only requires you not to interfere with free and ethical choices. Someone may want to use your car, but refusing to let them use it is not a violation of autonomy, because they can’t choose to use your car. They can only choose to ask permission use it. True, they can choose to use your car if it is unlocked and you leave your keys in it, but that choice would be unethical because it is illegal. Preventing them from getting into your car to drive it away would not be a violation of autonomy.
For additional explanation, watch the video Rational choice II (or read the transcript). It is part of an online ethics tutorial that consists of the following 30-minute sessions: