It's a good question. It's as clearly one he didn't expect to be asked. Isaac considers,

"Encouraged, is how I've always thought of it." Thought is its own shape, within magic. "But transfer is perhaps more accurate. There must be a tipping point. To which each of us, our life belongs elsewhere."

Something levels in the pitch of his brows, less performative. It would be easy to mistake for discomfort. A moment passes, is shrugged off. Light:

"If it's wet enough, you get mushrooms."