Unlike purely descriptive endeavors ethics is not only meant to describe what ethical behavior is but to provide us with reasons to be ethical (in technical terms we would say that an ethical theory should be normative). Many ethical theories claim that our desires are enough to motivate ethical behavior; meaning that if we want to satisfy our desires we should behave ethically. However many feel that theories such as these are lacking, for example it is hard to see how altruistic behavior could be mandated by an ethical theory, since by definition to act altruistically is to aid someone else when there is not benefit to yourself.
To address these concerns philosophers such as Thomas Nagel and Kant have constructed theories that demonstrate that it is irrational to act “immorally”. The sense in which rationality and irrationality is used in the context of these theories may be more restrictive than the common use of these terms. In short rational action and thought are based only on the principles of reason (such as deduction) and facts that are known or are likely to be true (their likelihood being deduced from other facts). Thus a rational person, defined in this sense, would not act on their intuitions or beliefs unless they had good reason to believe that they were true. Conversely to act on an unfounded belief or desire would be irrational. Below I will often use faith as an example of irrationality. This is not done to offend people, but because of the restrictive definition of rationality given here, faith, which is not based on evidence, fact, and deduction, cannot be classified as rational. Kant’s and Nagel’s theories build on this definition of rationality by showing that there is no rational reason to set one’s self above other individuals as a special case, and from this they derive their specific conclusions. For example Nagel defends altruism as moral because since we are treating all people as equal there is no reason to care more about your personal desires than those of other people, and thus helping other people to fulfill their desires, even if you aren’t benefited by it, is rational, while fulfilling your own desires while ignoring those of others is irrational.
As compelling as these theories are however their foundation is somewhat shaky, specifically they have a hard time addressing why one should be rational. Even if someone agrees with Nagel’s theory it is possible that they have no reason to prefer rationality to irrationality, in which case then it is expected that they will choose to act irrationally by satisfying their own desires at the expense of the desires of other people. Thus if these theories are to be truly normative they must provide a universal reason why rationality is preferable or motivating.
One possibility is that we, by our nature, prefer rationality to irrationality. Just as everyone is born with a survival instinct (well, at least in the vast majority of cases) it is conceivably possible that everyone is born with a rationality instinct. In some ways this account seems to fit with our experience, since psychological experiments have demonstrated that people have a natural need to give explanations for their actions. For example a person given a command under hypnosis will attempt to explain their later unusual actions rationally, even though there is no rational reason for them to have acted in that way. However, for the most part, people seem perfectly happy being irrational at least some of the time. For example the majority of the population is religious, but any belief justified by faith is irrational, as I mentioned above. Thus it seems unlikely that people are naturally driven to be rational since so many people seem happy being irrational.
Another possibility then is that rationality is necessary for the fulfillment of our other desires, meaning that rationality allows us to meet our wants better than irrationality does. Under this theory peoples’ desires in general will mandate that they be rational, even if they have no specific desire for rationality. For example let us say someone has a craving for sweets. It seems reasonable to suppose that rational thought will better aid them in finding and having some sweets than irrationality would. The problem with this account though is that, even if it was true that rationality was the best way to fulfill our desires, there is nothing that mandates that we be rational all the time. People could simply be rational when it was needed, behaving irrationally whenever their behavior isn’t directed at satisfying a want.
If we can’t show that there is a need to be rational perhaps we can show that people are motivated not to be irrational. Clearly there is no natural penalty for irrationality, since as mentioned above many people are happy to act motivated by faith instead of by reason. If nature doesn’t punish irrationality perhaps society does, for example our society might equate rational behavior with being human and irrational behavior with our animal nature. This would imply that there would be social sanctions against irrational behavior, thus providing a strong motivation to be rational. Unfortunately this account doesn’t reflect our actual society, where rational and irrational behavior are treated fairly equally in most cases.
It seems then that we can’t actually show how people are intrinsically motivated to be rational. If this is the case then an ethical theory based on rationality is not universally normative, and thus not really an ethical theory under some accounts of what ethics is. Of course if the idea of a conditional ethical system seems acceptable, that is an ethical system that only applies to people with certain motivations, then such accounts can be seen as an ethical system that applies only to rational people. This would explain why many philosophers are naturally drawn to such accounts, since most philosophers feel a strong desire to be rational, but unfortunately it doesn’t justify insisting that anyone else should act in this way.
You can read more on Nagel’s rational morality in The Possibility of Altruism (amazon).