I’d go with “I don’t think this conversation is helping us ________” (eg, come up with solutions, figure out what to do next, understand why we really disagree). It opens the door to suggest meta-level changes to how you’re interacting without 1) accusing them, 2) suggesting that they’re the only one who should change, or 3) inviting arguments about their intentions or internal state. It might also help reorient the conversation from a small-scale win/lose frame towards a broader shared goal of accomplishing something. Finally, it allows you to suggest specific changes to make the conversation more truth-seeking while also addressing whatever may be motivating them to not be truth-seeking (eg, fearing that if they “lose” then their needs won’t get met, perceiving you as trying to pull status on them, trying to preserve their image for an external audience).
If you’re having a debate that’s mostly for an external audience, then maybe you should just call out that the liar is lying. If you’re trying to work with them, then it’s probably better to try to figure out what aspect of the conversation is motivating them to lie and trying to incentivise telling the truth instead. If you can’t do that then it doesn’t really matter what’s going on in their head; you’re not going to have a productive conversation anyway.
Agreed that this would clarify a lot. On the other hand, if you’re very confident that taking the capsule helps, then at worst you’ve stumbled on the cheapest and least harmful placebo known to humankind (a sugar pill!!). Maybe you should just run with it (acknowledging that it helps while releasing or holding lightly to the proposed mechanism).