Quote from David in The One with the Donor
Phoebe: What are you doing here? David: Well, I'm back from Minsk. Permanently. Phoebe: Well, what happened? David: Well, remember how I was trying to achieve the positronic distillation of subatomic particles? Phoebe: Yeah. David: Well, after eight years of research, I discovered that ... that it can't be done.