Deontology is an ethical theory that says actions are good or bad according to a clear set of rules.

Its name comes from the Greek word deon, meaning duty. Actions that align with these rules are ethical, while actions that don’t aren’t. This ethical theory is most closely associated with German philosopher, Immanuel Kant.

His work on personhood is an example of deontology in practice. Kant believed the ability to use reason was what defined a person.

From an ethical perspective, personhood creates a range of rights and obligations because every person has inherent dignity – something that is fundamental to and is held in equal measure by each and every person.

This dignity creates an ethical ‘line in the sand’ that prevents us from acting in certain ways either toward other people or toward ourselves (because we have dignity as well). Most importantly, Kant argues that we may never treat a person merely as a means to an end (never just as a resource or instrument).

Kant’s ethics isn’t the only example of deontology. Any system involving a clear set of rules is a form of deontology, which is why some people call it a “rule-based ethic”. The Ten Commandments is an example, as is the Universal Declaration of Human Rights.

Most deontologists say there are two different kinds of ethical duties, perfect duties and imperfect duties. A perfect duty is inflexible. “Do not kill innocent people” is an example of a perfect duty. You can’t obey it a little bit – either you kill innocent people or you don’t. There’s no middle-ground.

Imperfect duties do allow for some middle ground. “Learn about the world around you” is an imperfect duty because we can all spend different amounts of time on education and each be fulfilling our obligation. How much we commit to imperfect duties is up to us.

Our reason for doing the right thing (which Kant called a maxim) is also important.

We should do our duty for no other reason than because it’s the right thing to do.

Obeying the rules for self-interest, because it will lead to better consequences or even because it makes us happy is not, for deontologists, an ethical reason for acting. We should be motivated by our respect for the moral law itself.

Deontologists require us to follow universal rules we give to ourselves. These rules must be in accordance with reason – in particular, they must be logically consistent and not give rise to contradictions.

It’s worth mentioning that deontology is often seen as being strongly opposed to consequentialism. This is because in emphasising the intention to act in accordance with our duties, deontology believes the consequences of our actions have no ethical relevance at all – a similar sentiment to that captured in the phrase “Let justice be done, though the heavens may fall”.

The appeal of deontology lies in its consistency. By applying ethical duties to all people in all situations the theory is readily applied to most practical situations. By focussing on a person’s intentions, it also places ethics entirely within our control – we can’t always control or predict the outcomes of our actions, but we are in complete control of our intentions.

Others criticise deontology for being inflexible. By ignoring what’s at stake in terms of consequences, some say it misses a serious element of ethical decision-making. De-emphasising consequences has other implications too – can it make us guilty of ‘crimes of omission’? Kant, for example, argued it would be unethical to lie about the location of our friend, even to a person trying to murder them! For many, this seems intuitively false.

One way of resolving this problem is through an idea called threshold deontology, which argues we should always obey the rules unless in an emergency situation, at which point we should revert to a consequentialist approach.

But is this a cop-out? How do we define ‘emergency’?

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Should we always live by the rules?