It a situation where different people are seeking to fulfill the most and strongest of their desires, there is good reason to seek approval from those whose desires are most intimately affected by an action before proceeding with that action. In morality, in many cases, consent is necessary for an action to be permissible.
Desirism and Consent
We are building on the proposition that desires are the only reason for action that exist. People have reason to act so as to objectively satisfy the most and strongest of their desires. This is accomplished in part by using social tools such as praise and condemnation to promote those desires that tend to objectively satisfy other desires and inhibit desires that tend to prevent the objective satisfaction of other desires.
Each of us has many and strong reasons to have others obtain our consent when acting in ways that affect our interests. We have reason to make others averse to performing those act-types when they do not have our consent.
We have no reason to prohibit these act-types at all times and in all circumstances (to prohibit people from having sex with us or with using our property). Instead, we have reason to make them averse to the act type when they have not obtained our approval.
At the same time, they have reason to make us averse to certain act types where we have not obtained their consent - to make us into people who just are not comfortable with taking their property when we have not obtained permission to do so.
Speaking about promoting an aversion to certain states is not the same as having a rule against it. A rule against eating donuts can be easily broken. An aversion to eating donuts means that an agent can be trusted not to eat donuts unless the situation is quite dire. The same applies to a rule against using the property of another without consent, as opposed to an aversion to using the property of another without consent.
The tools for promoting such an aversion includes acts of condemnation aimed at those who use property without consent. This sets up a biological feedback mechanism that not only promotes the aversion in the person condemned, but in others who are a witness to the condemnation.
The Most Knowledgable and Least Corruptible Agent
By definition, each of us can promote the objective satisfaction of more and stronger of our desires by giving the task of deciding what to do to the most knowledgable and least corruptible agent available. This decision maker would be the person who has the most information and the fewest distractions that would otherwise move them to ends that would not objectively satisfy the most and the strongest of our own desires.
For the majority of us, the most knowledgable and least corruptible agent to task with directing our own lives is ourselves. Corruptibility is not even possible - we always act so as to fulfill the most and strongest of our own desires (given our beliefs). We are never sacrificing our own interests for the sake of another, because doing so requires a stronger interest in the welfare of others.
Note that this is not the same as the claim that we are all selfish. Selfishness holds not only that we always act on our own desires, but that the only thing we desire is our own well-being. A person who sincerely desires the well-being of others - for whom this is the most important thing in life - is not selfish.
An agent's own actions comes from that agent's own desires as a matter of necessity - actions that did not spring from one's own desires are one's own actions.
If an agent's actions go awry, it is because of false or incomplete beliefs. Yet, even here, the agent has spent a lifetime doing research on the facts relevant to the objective satisfaction of that agent's desires. In general, an agent knows more about things that matter to that agent than any other person.
Where actions go wrong, it is generally a result of false beliefs. The act selected is one that would fulfill the most and strongest desires in a universe where the agent's beliefs are true. However, the agent's beliefs are not always true. Consequently, an act that would objectively satisfy the agent's desires in such a universe will not necessarily satisfy those desires in this universe.
Still, for the most part, the agent with the fewest false beliefs and certainly the least possibility of corruption when it comes to making decisions for an agent is the agent herself.
Even when agents are clearly wrong on some matters - if, for example, some cult leader has filled their heads with false beliefs about an afterlife and what they must do in this life to get it (e.g., serve and obey the cult leader) - it is still safer to trust each person to direct their own life then to eliminate this aversion to consent - and risk giving the cult leader dictatorial control over the lives of others.
In general, each of us has a reason to plant in others an aversion to actions not approved of by the most knowledgable and least corruptible authority on our interests. For each of us, that authority is ourselves. At the same time that each if us has reason to promote an aversion to act -types without consent, others have reason to promote the same aversion in us. Our interests - those things that give us a reason to act - to praise and condemn - comes to include this aversion to act-types performed without consent.
The Consent of the Governed
We do not have a reason to promote universal consent to every single act. In fact, this is an incoherent option, given that we would then need to obtain permission to ask for permission. Thus, the moral role of consent is not universal and absolute. It applies to the most personal and intimate acts by those directly affected. There is a reason to put limits on how far out to go with this aversion.
In some cases, in order to make a decision efficiently and yet to obtain as much consent as possible, we use a proxy for consent. "Let's see if we can efficiently get above a certain minimum threshold of consent - let's say, more than half the people." We take a vote. Governments get their legitimacy from the consent of the governed, but not the unanimous consent of every single citizen.
This corresponds to promoting an aversion to thwarting the will of the majority - an aversion that people generally have a reason to promote because it avoids civil violence. An aversion is not an absolute prohibition - such as in cases where the majority becomes tyrannical, but it creates a disposition that "governments long established shall not be changed for light or transient causes."
Guardians and Wards
Some agents are not the most knowledgeable when it comes to their own interests.
This is the case with children and some adults who are mentally impaired. They lack the opportunity or ability to form the true beliefs relevant to objectively satisfying their own desires.
In these cases, assigning their decisions to the most knowledgeable and least corruptible agent means assigning those decisions to somebody else. Typically, we have reason to believe that a close relative is the person both with the most knowledge and least corruptible interest in the well-being of somebody who lacks competence. We prefer to give decision-making capability to the parents of a child, for instance. However, when evidence shows that the parent either lacks knowledge or sufficient concern for the interests of the child - or has strong desires incompatible with the interests of the child - we may deny that they fulfill the requirements to be a decision-maker for that ward.
The guardian's duty is to act as a proxy for the ward in making decisions - to make the decision the ward would make if the ward were appreciated the relevant facts.
Every situation where one agent controls the life of another is exploitive to some extent. The guardian will always act to fulfill the most and strongest of the guardian's own desires (given his beliefs). We can seek a guardian whose own desires include strong desires compatible with the interests of the ward, and few desires incompatible with the interests of the ward. However, we can never find an ideal guardian. He will always have some interests incompatible with the interests of the child, and will certainly suffer from a lack of knowledge.
Another important element of consent is the fact that we deny that a person gave consent when a person acts under duress.
We have some preliminary work to do before we look at the duress directly.
Agents act so as to objectively satisfy the most and strongest of their desires (given their beliefs). Let us assume that Person A has a desire that P and Person B has a desire that Q. One way that Person A can realize a state of affairs S where P is true is to tell B "S or not-Q". That is to say, "Either you act so as to objectively satisfy my desire that P, or I will act to prevent the realization of any state in which Q is true." B, with a desire that Q, now has a reason to act so as to realize S.
Under this description, we cannot actually tell whether A is treatening B, or whether A is offering a transaction that B has reason to accept.
Our description fits a case in which A tells B, "Either give me $100 or I will break both of your legs." In this case, B gives A $100 to prevent the realization of a state to which B is averse – a state in which both of his legs are broken.
It also fits a case in which A tells B, "For $100, I will drive you to the airport." In this case, B gives A $100 to prevent the realization of a state to which he is averse – a state in which he misses his flight.
Why is the first case an example of extortion, and the second case an example of a legitimate market transaction?
The most obvious difference is that the first option requires an action that a person averse to making another worse off would not perform. In the second example, not giving B a ride to the airport leaves him worse off than if he got a ride. However, A is not responsible for B being worse off. B would also be worse off in a universe where A did not exist.
People generally have many and strong reasons to promote an aversion to making others worse off. We are better secured in a community surrounded by people with such a strong aversion to making others worse off that we can trust that they (or the vast majority of them) will never violently harm us. People have many and strong reason to promote this aversion by condemning those who lack it - even where the person they condemn are fictional people in a story meant to teach a moral lesson.
Threats are actions that a person with a proper aversion to making others worse off would hate to perform. They are actions that invite condemnation.
We could have invented a language with two different types of consent - morally legitimizing consent and consent under duress. An argument that consent under durress is not morally legitimizing would still look at the fact that a person with a proper aversion to making others worse off would not perform such an act - that people have many and strong reasons to condemn those who lack this aversion.
Though we sometimes see hints of this language, for the most part we have adopted a language where we use the term "consent" only in the case of morally legitimizing consent in the language mentioned above, and deny that the term applies in cases where threats are used. These are simply two different languages that describe the same phenomenon. The phenomenon is one where consent morally legitimizes an action unless it is accompanied by a threat.
Asserting that one had obtained consent implies moral legitimacy. We have built this into the meaning of the term - and then bounded the term by what people generally have reason to condemn.
People have a reason to promote an aversion to acting without consent when agents are the most knowledgable and least corruptible guardians of their own interest. When these conditions do not apply, we deny that consent is even possible.
People have a reason to promote efficient proxies to consent for large groups. This involves promoting an aversion to going against the will of the majority (within limits). Therefore, when a vote is taken and the group, by and large, agrees to a proposal, we talk about this in terms of the "consent of the governed."
People generally have reason to condemn the lack of an aversion to causing harm that makes threats possible. Consequently, we deny that consent is obtained when a threat is used.
This means that the concept of "consent" carries a lot of morally loaded baggage that a proper analysis of the term must unpack. Yet, it leaves us with a concept where, in many cases, if a person obtains consent, an action that would have otherwise been morally prohibited (an act that a person with a proper aversion to acting without consent would not perform) becomes permissible.