So, when you read Paul Craig Roberts and Paul Krugman, you do so without caring about their motives? Both those guys are putting out a lot of correct arguments these days. You don't wonder how and why?
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Not nearly as much as I should wonder how and why when Bush says the US is going to invade Iraq because they possess WMD, and then it turns out they don't have any and that Bush knew they didn't.
I think it's possible to gain a greater understanding of a person you agree with by analyzing his/her motives. But assessing arguments and assessing motives are two distinct things. Moreover, I commonly turn to motives when arguments fail the rational test, but the person I'm arguing with persists in them. That's why ascribing motives is usually considered insulting, and why the whole thing is such a hot-button issue.