Derek Parfit.

During arguments we often hear people say, ‘You’re being irrational!’ The accusation is that the person being spoken to is failing to act in line with their best interests. However, according to Derek Parfit (1942–2017) in his classic text of contemporary utilitarianism, Reasons and Persons, this isn’t always a bad thing.


Parfit discusses ‘Self-interest Theory’ at length, the thesis of which is that we have most reason to do what we believe will work out best for ourselves. As such, it’s irrational to act in ways which we believe will make matters worse.

Parfit believes Self-interest Theory ultimately fails, using various examples along the way to show how rational acts aren’t always the best options.

The writer

Kate’s strongest desire is to write a good book. But she knows she might end up overworking herself (relatable), causing her exhaustion and depression.

She chooses not to write it (unrelatable).

Was this the right choice? Kate’s motives are rational on account of her hedonistic views: there’ll be less pain than pleasure. However, her life is bigger than weighing up pain and pleasure, which her rationality fails to identify.

The robber

A man breaks into my house. He orders I empty my safe of valuable possessions for him.

What shall I do? My family and I have seen the man’s car and its licence plate. This makes handing over my possessions irrational because he’ll probably kill us anyway. However, he’s threatening to kill my family if I don’t, which makes not doing so also irrational. There are no good rational options: I’m stuck.

Parfit suggests I drink a special solution conveniently sitting on the worktop next to me. It contains an irrationality-inducing drug. I drink it.

The man threatens me. But now I’m simply crazy—laughably so. ‘I love my children. Please kill them!’ I say. ‘Please carry on hurting me!’ The man loses his power over me, since I’m no longer rational, and flees.

In conclusion

Rationality is usually associated with guiding us towards our best interests. But it fails. And, as Parfit shows—albeit outlandishly—it pays to be ‘rationally irrational’.

Note: It’s not always distinctly clear what the magic mode of thought we call ‘rationality’ is meant to be. ‘Best interests’ in our definition is doing a lot of work here, for what are they?! Someone can always provide valid reasons for their actions, however harmful they are. For example, I might ask for my arm to be amputated because ‘I don’t like it anymore’. I might also watch my favourite football team play week upon week, despite how awful they make me feel on average, ‘to feel a connection’. There is nothing illogical per se about my motive in either case.

Also, according to a desire-fulfilment theory of reasons, what’s better for Kate is for her to fulfil her strongest desires. Will this enable Self-interest Theory to work?