Intertemporal and Interpersonal Reasons

This is another post basically just making note of an interesting section of Reasons and Persons.

"Intertemporal" means "occurring across time". We basically all accept that reasonable decisions must account for their effects on ourselves across time. If someone chooses to jump off their roof because they enjoy the sensation of falling, and then they feel miserable when 2 seconds later they break their leg, we would consider there to be something seriously wrong with them. Such a person could argue "but at the time I chose to jump it was enjoyable; why should I have cared at that enjoyable moment that my future self would be in severe pain?" We would consider that argument to be irrational and that person to be mentally unstable.

"Interpersonal" means "occurring across people". We mostly accept that taking interpersonal affects of our actions into account is covered by morality but not rationality. In other words, while we may claim that someone is morally wrong to make a selfish decision that favors themselves at the expense of others, we don't think it's necessarily irrational to do so.

But why? Why would reason extend across time but not across people? Derek Parfit (the author of "Reasons and Persons") makes the case that we should think about these the same, which seemed absurd to me at first, but after further thinking (and further explanation in the book), it has started to make a lot of sense. He claims that we should see good intertemporal decisions as under the realm of morality rather than rationality. It seems like you can make the reverse claim just as well: immoral actions are irrational. We may be instinctively inclined to care more about our future selves than about other people, but both my future self and another person matter equally from an objective/rational standpoint.