6 ideas
Some experiences are epistemically transformative. You can only learn what they are like by having them, the way Mary, who has spent her life in a black-and-white room, cannot know what seeing red is like until she sees it. Before the experience, the subjective values of its outcomes are not merely uncertain but unavailable to you, because you lack the concepts needed to represent them.
An experience is personally transformative when it changes your core preferences, values, and self-conception, so you become a different kind of person. That means the self who evaluates the choice beforehand and the self who lives with its outcomes may rank those outcomes differently.
Standard decision theory tells you to assign a subjective value to each outcome, weight it by probability, and choose the maximum. When a choice is both epistemically and personally transformative, you cannot assign those values from the first-person perspective.
Suppose you are offered the chance to become a vampire, and every vampire you know reports loving it. You still cannot know what being a vampire would be like for you, and your vampire self would have different values from the ones you hold now. The thought experiment shows that neither testimony nor imagination closes the gap, which makes the choice hard to justify either way.
You could replace your own imaginative projection with third-person data, such as surveys of parents' happiness. Doing so gives up the authentic, first-personal deliberation we prize, in which you choose based on what the experience would be like for you. The data also averages over people whose preferences may not match either your current self or your future self.
Paul's proposed resolution is to reframe a transformative decision. Instead of asking whether you will prefer the future outcome, you ask whether you want to discover who you will become and what the new experience reveals. The value being chosen becomes the revelation itself, which you can assess now, rather than the unknowable subjective quality of the life on the other side.