Many of our choices affect the future. Some choices, like deciding to build a bridge or a power plant, have consequences that ripple out for generations. Most of the time, we judge these choices by a simple standard: did they help or harm people? But what if a choice determines who those future people are? This question leads to a thorny philosophical puzzle known as the non-identity problem.

The problem arises when an action determines both the quality of life for future people and the very identity of those people. It challenges our basic ideas about harm and moral responsibility.

Let’s start with a classic example. Imagine we have to make a major decision about our nation's energy policy. We have two options:

1. Depletion: We continue to burn cheap, polluting fossil fuels. It’s easy and keeps energy costs low for us now, but we know it will cause significant environmental problems in 200 years, leading to a much lower quality of life for the people living then.
2. Conservation: We invest in expensive, clean energy. This requires sacrifice today, but it ensures that the world in 200 years will be clean and people will have a much higher quality of life.

Intuitively, Conservation seems like the right choice. Depletion seems selfish and wrong. But here’s the twist: the people who will exist in 200 years are different depending on which policy we choose.

The energy policy we enact will cause massive social and economic changes that will ripple through time. It will change who meets whom, who marries whom, and when they decide to have children. The timing of conception is critical to an individual's identity. If a couple has a child a month earlier or later, they have a different child because a different sperm would have fertilized a different egg.

So, the people born into the polluted world of the Depletion scenario would never have been born if we had chosen Conservation. Let's call them the "Depletion people." Can they complain that our choice harmed them? If we had chosen Conservation, they wouldn't be living better lives; they wouldn’t be living at all. Someone else would.

This is where our moral intuition hits a wall. The wall is a widely held belief called the person-affecting principle.

Let’s apply this to our environmental case. The Depletion people are living in a polluted world, but let's assume their lives are still worth living. They have struggles, but also joys. Are they worse off because we chose the Depletion policy?

No. The alternative for them was not existing at all. Since having a life worth living is better than no life at all, our choice didn't make them worse off. According to the person-affecting principle, our choice wasn't wrong. Yet, that feels deeply incorrect. We've created a worse world, but we haven't harmed anyone in it.

The problem gets even clearer with a smaller-scale example. Imagine a 14-year-old girl who decides to have a child. She knows that because of her young age, she won't be able to provide her child with a good start in life. The child will likely grow up in poverty and face many disadvantages. If she waited ten years, she would be mature and financially stable, and she could give a child a much better life.

She decides not to wait. She has a child now. As predicted, the child has a difficult life, but it's still a life worth living. Was the mother's decision morally wrong?

Let's ask the child. If his mother had waited, he wouldn't have been born. The child born ten years later would have been a different person, his sibling. So, the choice wasn't bad for him. His mother's choice gave him the only life he could have. It seems we can't condemn her choice on the grounds that it harmed her child.

This is the non-identity problem in a nutshell. We have a strong intuition that creating a worse world or giving a child a bad start in life is wrong. But when the action in question is the very thing that creates the person, it's hard to argue that the person has been harmed. This forces us to question whether the person-affecting view is the whole story when it comes to morality.