Nod. There's knowing what they do and knowing how to imitate.

The fact that he's trying that argument is a surprisingly good sign. If he were currently adversarial he could just not bring that up and walk off with the information.

Somehow I bet we could figure it out. Maybe not in a month. I'd put something-materially-scarce down on having it within three, though, and certainly on the time scale where we might end up our cousins' adversaries.

I'd be tempted to take that bet, but kind of already did by giving you the books.

So, the way I see it, I can pretend we haven't figured it out and thereby avoid committing our resources and - to the extent things are zero-sum between our hosts - harm you, or I can admit I've figured it out and then start helping even though this will probably make my reputation for untrustworthiness even worse. And I'm going to do that, because I want the Enemy dead very badly.

Or you could tell us and we could spend the research time on something else, but I'm not sure the research won't be fruitful even to the extent it doesn't bear on this specific secret plan of yours, and we'll probably accomplish about as much magical-research-wise in the next year whether we're hunting down your secret or not. I am assuming it's not reproducible from the effect, or the Enemy can do it, but it's also not tremendously complex because it's not in your domain of specialization and you don't know much else that is complicated and you're not bottlenecked by training people to do it. I can brute-force that in a year, the way my people work. 

So the only advantage to telling us now is that we can start helping sooner - and, I guess, that we can't admit we know it and offer our help at a time convenient to us - but it certainly seems that the only disadvantage is that you don't like us, unless I am to conclude you're planning on picking a fight with us in the next few years.

We definitely aren't. 
Didn't booby-trapped the books or anything, by the way, that's the same translation we use.

I'm not going to comment on the difficulty of duplicating this plan. But if you do then I don't think saying you did will hurt your reputation more; we know you have spies and you haven't agreed not to try.

All right. Let us know if you change your mind on wanting help before we've got it. You'll know when we've got it, because it obviously involves a way of interacting with practitioners at range or it wouldn't be a new development.

Well, that sounded ambiguously threatening. I assume you didn't mean it at face value.

Far be it from me to sound ambiguously threatening. If you can get me an estimate of how valuable being able to trust us on this would be I can see if I can get clearance to be outright helpful.

On this in particular, helpful but not critical. Less valuable than being able to trust each other in general would be.

I know. If there were a less costly solution I'd do it in a heartbeat.

I believe you.

Good luck with the research, I suppose– you'll be advanced enough soon that the warnings about trial and error start applying, and exceeding your current skill is kind of the point...

Take care. Do let us know if it's urgent you trust me.

I really hope that doesn't become urgent. But I will.

Take care.

If it were, what I'd do is swear not to keep any thoughts private from you. But  - the trust problem goes both ways, on something like that.

I, uh, very definitely hope it doesn't become that urgent. That's terrifying.

Luckily we have the lovely alternative of me working around your disinclination to tell me things. 

Keep in mind if you don't know this specific thing we're not worried about you overwhelming us with magic. Might not be the best use of a month's spying resources even if it'd work.

If it'd be worth having our help with, enough that you came out here hoping we'd think of something that'd let you trust me, then it's worth having our help with even though I can't think of anything without unacceptably high costs.

I don't think that follows. Playing this card close to the chest means we can be more confident about giving out other magic knowledge; if there were something unexpectedly convincing we could have done both.

Maybe I just stubbornly want to be involved in anything at all that hurts the Enemy, if the opportunity costs are worth it by your estimation and the only reason it's not happening is that you don't trust us.

Could be. If you did it would even help. The question is whether it'd help more than us continuing to share other magic usable against the Enemy.

I don't think I understand why our figuring out this makes you less able to share other things.

Less safe. It would be more effective if one of our hosts used it against the other than against the Enemy. If we have it and you don't we're almost categorically not worried about open attack.

If you were serious about not particularly wanting to keep that option open, and think you can trust us not to strike first of course, not pursuing this is a pretty good way to go.

I'd much rather achieve that result by having us all swear to drop any weapons and not use any magic if you say 'elephant' than by not picking up things that'll help against the Enemy.

That sounds stupidly dangerous. If nothing else, the Enemy will probably have better impersonation ability soon.