What is a human:
If the human mind could swap bodies it would not matter. So only the mind matters.

But most of what the mind does is not important in the long run. E.g. most eating, working, sleeping, using the toilet, etc., are just means to an end. The end itself, the important part, is our values and effect on others. So only your values (and effect on others) matter. So the real you is your values and effect on others, nothing else.

So the real you is shared every time you interact with others. So the real you lives though multiple bodies. You even share some values with animals, plants, and objects like the stars. For example, Pythagoras once recognised a dead friend's personality in a dog's bark.

It might help to remember that time does not flow. So the future "you" is a different person. So there is no advantage in sharing yourself with only that person while excluding all the others.

This is the basis of morality. If you live for just your current body then you become selfish, and die when your body dies. But if you see your body as just a tiny part of you, then you focus on others then you spread and live on when your body dies.

Pythagoras illustrated this through the humble bean. If you think of a bean as a single bean then you eat it and it is gone. But if you think of a bean as its future network then you plant it, and while that bean body may have gone, it is now unlimited beans!

So the key to happiness is to see yourself as a bean!

About Pythagoras
What is theology?
What is religion?
Why do things exist?
What is time?
Higher intelligences
The simulation argument
Anything else?