"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? 

Yep!

"Cool!"

Heidi grins. 

Can she touch her nose?

With absolutely no sensory feedback, but yes she can.

"I just tested proprioception by touching my nose and couldn't feel my nose." And realized that she may be supplementing her proprioception with Mary's secondhand sight. 

"Mary?" She asks. "Could you turn around?"

Heidi, that's not how it works, if you make a falsifiable test and then expect the test to fail it's going to fail, he just told us his power works like forcing. 

 I was going to suggest that you try forcing me through a non-visual sense. This is a physical, corporeal body. I will not disappear when you are not looking, so you won't have to visualize me. 

I'm just scared that if I try to hard to not perceive you you'll disappear, or --

No. No. You're real, this is what we're proving, right now, that you're real even when I'm not looking.

Yes.