A Claim to Motive
Also known as: Ulterior Motive
The motives of someone making a statement are not automatically relevant to the truth of the statement. The statement might be true or false, regardless of the motives of the speaker. If the statement is knowingly wrong, their motive might reveal why they made it anyway, but the statement must first be disproved on its merits.
Examples
Wealthy people often argue in favor of a tax cut as a way to achieve a healthier economy. Clearly, the more income one has, the more a tax cut will benefit them, so these arguments are often disregarded as a rich person wanting to protect their income. However, just because the possibility exists for another motive, does not mean their argument for lower taxes is wrong.