They call it "souls" here. He needs souls to strengthen himself. Other demons feed off souls, or try to steal souls to power themselves. But that's rubbish. The process can be simulated almost identically with what we know of Jiaying's abilities. [With what he knows of her abilities, after having studied her at the atomic level in the Framework, to give Ophelia the longevity he thought she deserved. The Doctor was never a stranger to seeking a new life in another world.] It's -- something I've had to look into, once. Part of blocking his father out. It would've killed him to ask him to sustain that kind of block on his own.

[He sighs.]

For all the good it did us in the end.