Miles Jackson : Call me a well indoctrinated child of the Enlightenment, but the motives of someone who is providing information or a specific argument are completely irrelevant to a critical and rational assessment of the person's assertions. If you say "Social security funds will be completely exhausted by 2042", speculating about your motives is a waste of time. Whether you really believe this and are trying to spread the truth or you are disseminating lies to further your political agenda, the question remains: Is this in fact a valid statement?
Knowing your motives doesn't answer that question; only careful and rational consideration of the available evidence can do that.
^^^^^^ CB: What you say is true. However, the problem is there is no evidence of the status of social security funds that can be gotten from anything other than people. So, if those people lie or are biased, your available "evidence" might be false.
Thus in court, a witness's biases , and thus motives,are pertinent in judging the reliability of her testimony. It is not an _ad hominem_ fallacy to present evidence of a witness's bias, i.e. motives.
Example: Cordelia says "Charles didn't do it." Cross-examination of Cordelia: "Isn't it true that you are Charles' mother ? " Objection. Objection overruled.
Similarly, if I say, "George Bush's claim that Social Security is failing is not reliable , because he is a rightwinger trying to destroy the New Deal " , I do not commit a logical fallacy, _ad hominem_ or otherwise, but rather make an important contribution to the investigation of the status of Social Security.
In other words, evaluation of the human sources of information is part of careful and rational evaluation of what is reliable evidence. As Doug mentioned here, and I have said before, _most_ of what we all know comes to us through many layers of hearsay, that is other humans, and evaluation of the reliability of those human witnesses is very important in evaluating evidence.