"Lots of people react with doubt, we can take it. Although I would prefer if you act as if Heidi exists around me, even if you don't believe it yourself."

"I mean, uh... I'd like to believe things that are true, and this is an interdimensional bar so I'm pretty open-minded about what things could be true, it isn't that hard to believe there's someone else in your head. I just really don't know if I could do it."

"And if you don't think you can do it, then you won't. On the other hand, if you do think it's possible, it is."

"...um. There is presumably a fact of the matter as to whether a brain of a human from my world can house more than one person simultaneously, or create a new one, like that, that is independent of my belief, I'd think."

"Why do you think it's independent of your belief?"

"Because a brain is a machine that's running an actual software and has an actual capacity. I, uh, maybe in your universe there are gods? Other than Scion if that I'm pretty sure mine is godless and soulless so there's a finite capacity to a person's brain and whether it fits two people in it and can run them should be a deterministic question with a deterministic answer that depends on its physical characteristics and nothing else."

"In our world we have a mental illness called dissociative identity disorder, which looks similar to having a tulpa except that it's involuntary. And if another personality fronts, the host blacks out -- that doesn't happen with tulpae. Do you have that in your world?"

"Yeah, we do have that, though I think the evidence on whether it really exists is mixed. But I know that people can be full people after a hemispherectomy so it sounds plausible that two people could inhabit a brain, it's just that creating a whole new person sounds... well, difficult, at best."

"Well. It depends on how difficult you find modelling other minds -- it took a while for us but it wasn't that hard.

I suppose I'm a bit biased, having done it."

He grins. "I suppose so, too. But, knowing that it's a perfectly natural and nonmagical process, why would you expect it to work on an," he gestures, "empty construct?"

"I suppose I don't know that it would work -- my belief in Heidi's existence doesn't necessarily interact with your power."

Well...

"Did you expect it to?"

"I didn't know how your power worked! You said it was unlikely, but I thought it would be worth a shot, and you said you hadn't tried it before."

"Yeah, fair enough, I suppose." He looks around at the otherwise empty bar then says, "So I have a secret and it'd be very dangerous to me if it escaped to my world but you're not from there anyway, and also I'm pretty sure now it's the reason the door picked us."

"Oh?"

"The, uh, mental state I mentioned, is... belief. Kinda. Expectation, really. I have to expect a thing to happen, and it will. But the trick to it is that other people's beliefs also count. If I tell you I can conjure a thing, and you actually expect me to be able to, it's much easier for me to do it."

Oh. 

... Oh. 

Mary immediately expects a rainbow ping-pong ball to appear outside her space as hard as she can. 

Nothing happens.

Hmm.

"I'm going to conjure a ping-pong ball right over there." She points to a spot outside her range.

Then she forces the ping-pong ball. 

There it is!

"That wasn't me conjuring right then -- that was you. The ping pong ball was outside my range.

I really think conjuring Heidi a body would work."

"Then let's try it again."

They do!

This time Mary forces Heidi instead of relying on Sadde to do it himself. 

There she is.

Can Heidi move her arm?