I mean to say: "because it would be in the best interests of claimant Y for claim X to be true, [and because I have proved X is false], I suspect that Y is lying." In short, once you establish that X is false, you can then use that fact to impugn motives. You cannot do it the other way around. The fact that Mr. Y says X is never proof that X is true. Strictly speaking. In real life it is common sense to suspect that Mr. Y. may be motivated by crass self interest. Go ahead and suspect that, but just remember it is a logical fallacy.
>From the website: "A person's interests and circumstances have no bearing on the truth or falsity of the claim being made. While a person's interests will provide them with motives to support certain claims, the claims stand or fall on their own." - Jed
