The questioning of motives is one of the most common and most pernicious of rhetorical habits in political debate. It's pernicious because it encourages people to conclude not that your opponents are wrong about whatever matter it is we're discussing, but that they're bad people. When you question someone's motives you're automatically calling them a liar (since they will have offered an entirely different justification for why they are advocating what they're advocating), and you're also saying they're untrustworthy, cynical, and driven by some nefarious goal.
We see this all the time, and I'm not saying I've never questioned anyone's motives, because from time to time I have. But we have to acknowledge that someone can take a different position from the one we do without the disagreement coming from some place of evil. To see what I'm talking about, here's today's column by Charles Krauthammer, probably the most admired columnist on the right. Appalled that President Obama is now running for re-election and disagreeing with his opponents on matters of policy after saying he would try to unite the country, Krauthammer says this:
The entire Obama campaign is a slice-and-dice operation, pandering to one group after another, particularly those that elected Obama in 2008 — blacks, Hispanics, women, young people — and for whom the thrill is now gone.
What to do? Try fear. Create division, stir resentment, by whatever means necessary — bogus court challenges, dead-end Senate bills and a forest of straw men.
Why else would the Justice Department challenge the photo ID law in Texas? To charge Republicans with seeking to disenfranchise Hispanics and blacks, of course.
You'll notice that in the cases Krauthammer cites, the real crime is in Democratic responses to radical Republican initiatives. Depriving women of access to contraception is just good policy, but criticizing Republicans when they do so? How dare you. The real key here is in the words "Why else." Why on earth would the Justice Department challenge a voter ID law in Texas? Could it possibly be because in their judgment the law unfairly discriminates against certain groups? No, it could not possibly be. The idea is so absurd it isn't even worth mentioning. The only possible interpretation is the one that ascribes the greatest degree of cynicism and dishonesty to the people Krauthammer doesn't like, that their purposes are entirely political.
I'm not saying that on certain occasions it isn't reasonable to question someone's motives. In fact, voter ID laws offer one such case. The idea that all these Republican legislatures set out to address the non-existent problem of people impersonating other people at the polls just because they care so deeply about the integrity of the ballot, and did so in a way that purely by accident has the potential to significantly reduce turnout by some of the people most likely to vote Democratic, is more than a little hard to swallow. I'll absolutely grant that Democrats dislike voter ID laws primarily for the same political reason, because it means their voters may find it difficult to vote. But on the substantive merits, Democrats also happen to be right.
But on the whole, motive-questioning is rhetorical poison. It says to people, "Don't believe anything the other side says—in fact, don't even listen to what they say. All you need to know is why they're saying it, and the answer to that question is that they're evil." Now that's divisive.