The line of thought is that existence and non-existence are not comparable. The challenge in defending an asymmetric person-affecting view is arguing why it’s not good for someone to be creatied with a happy life, but why it is bad for them to have an unhappy life.
Maybe the first is good in a sense, but the goodness and badness should be thought of as moral reasons directed from outcomes in which they exist to (the same or other) outcomes, or something like world-dependent rankings. Existence and non-existence are comparable for an individual, but only in outcomes in which the individual actually exists (or comes to exist). You might imagine this like a process of deliberation, starting from one outcome/choice, and then following the moral reasons to others whenever compelled to do so. You would check what happens starting from each choice/outcome. To illustrate the procreation asymmetry, which is pretty simple:
There’s no arrow starting from Nonexistence, and the person who doesn’t exist wouldn’t rank any outcomes (or have outcomes ranked for them) precisely because they don’t/won’t exist. So Nonexistence is permissible despite the presence of Positive existence as an option, since from Nonexistence, nothing is strictly better; there’s no reason from this outcome to choose otherwise.
From Negative existence, Nonexistence and Positive existence look better, since the individual would rank Nonexistence better for themself, or this is done for them.
From Positive existence, Positive existence is ranked higher than Nonexistence and Negative existence and not worse than any option, so it is permissible. It is not obligatory because of 1.
Maybe the first is good in a sense, but the goodness and badness should be thought of as moral reasons directed from outcomes in which they exist to (the same or other) outcomes, or something like world-dependent rankings. Existence and non-existence are comparable for an individual, but only in outcomes in which the individual actually exists (or comes to exist). You might imagine this like a process of deliberation, starting from one outcome/choice, and then following the moral reasons to others whenever compelled to do so. You would check what happens starting from each choice/outcome. To illustrate the procreation asymmetry, which is pretty simple:
There’s no arrow starting from Nonexistence, and the person who doesn’t exist wouldn’t rank any outcomes (or have outcomes ranked for them) precisely because they don’t/won’t exist. So Nonexistence is permissible despite the presence of Positive existence as an option, since from Nonexistence, nothing is strictly better; there’s no reason from this outcome to choose otherwise.
From Negative existence, Nonexistence and Positive existence look better, since the individual would rank Nonexistence better for themself, or this is done for them.
From Positive existence, Positive existence is ranked higher than Nonexistence and Negative existence and not worse than any option, so it is permissible. It is not obligatory because of 1.
1 and 2 together are the procreation asymmetry.
I discuss this more here.