Ad Hominem argument/fallacy
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Original Question
It seems that calling "attacking the messenger, not the message" a fallacy argues that "all opinions are equal, independent of the source's competence, motive or expertise".
But given that everyone has an opinion, wouldn't judging the competence, motive and expertise of the messenger be a indispensable method in practical day-to-day life?
I won't take medical advice from total stranger seriously, unless he's a doctor. Is that a fallacy?
Answers
1Hello William,
The Ad Hominem (Abusive) is defined as attacking the person making the argument, rather than the argument itself, when the attack on the person is completely irrelevant to the argument the person is making. In your example, clearly a stranger giving medical advice is justification for doubt—but not necessarily an attack. The ad hominem is also often used as a diversion to avoid the issue, which increases it's fallaciousness.
The credibility of the source is very useful heuristic, but like all heuristics, the shortcut sacrifices a piece of your reason for cognitive ease. If we had the time, energy, and motivation it would be in the best interest of truth to evaluate each claim independently of the source. But that is not how the world works. Paradoxically, the lack of applied reasoning to every case makes us more reasonable. Ignoring the credibility of the source for every claim would result in far more problems than it would solve.
Hope that helps!
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