In this week’s #SkepticalTuesday, I explore just when it’s appropriate to look at the person making the claim, and not just their argument.

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Argumentum ad hominem (just ad hominem for short) is a Latin phrase used in English to describe the practice of attacking the character of one’s opponent, rather than her or his arguments. It’s generally considered a logical fallacy, because, after all, you’re not actually disproving what your opponent has argued, just making negative comments about your opponent. But is it ever not a fallacy?

This week, my facebook wall lit up with multiple iterations of a sensational headline: the surface of Comet 67P/Churyumov-Gerasimenko — the one Philae is sitting on and Rosetta is flying around — might have life.”The Philae Comet Could Be Home to Microbial Alien Life, Scientists Say,” read the headline at TIME. “Philae comet could be home to alien life, say scientists,” read the Guardian. “Does Rosetta’s comet harbour ALIEN LIFE? Distinctive features on 67P may have been created by microscopic organisms,” read the Daily Fail Mail.

Now if you’ve been reading the science news for the past decade — either because you do it for a living, or you’re a massive geek like myself — you’ll have raised an eyebrow, clicked a link, read the first few sentences, then lowered it again. Probably, after this point, you’ll have rolled your eyes and sighed before doing some more rigorous fact-checking on the topic.

Why? Because you read the name Chandra Wickramasighe.

To put it mildly, Professor Chandra Wickramasinghe, of the University of Cardiff, has a reputation for making poorly-supported, media-friendly claims about alien life. Phil Plait, over at Bad Astronomy, does a great job of detailing them while still being remarkably generous about it. From the time he said the flu virus comes from comet dust, to the time he said a red rainstorm in Kerala was due to alien spores, to the time he said the reason NASA wasn’t saying there was definitely life on Mars was due to politics — he’s had a good run. The common thread in all of these claims is the theory of panspermia — the idea that “life down here began out there,” as it were — of which Wickramasinghe is a major proponent.

And there’s nothing wrong with the theory of panspermia. Our growing body of research on extremophiles, not to mention the presence of organic (not necessarily living!) compounds on an ever-longer list of extra-terrestrial bodies, has reaffirmed time and again that it is definitely possible that there is life out there. But until we actually find it, get a good close look at it, and determine whether it’s pretty much identical to Earth life, we aren’t going to know. Heck, even then we might not know, because if panspermia is possible, then it could be Earth seeding the rest of the planets (although gravity being what it is, it’s easier for things to move toward the sun than away). But the fact of the matter is: we don’t know. We can’t know. Not yet.

But as Plait has so eloquently pointed out, Wickramasinghe “is a fervent proponent of [panspermia]. Like, really fervent. So much so that he attributes everything to life in space.”

And this is where ad hominem crosses over into source analysis. If, every time something unusual happens, you claim that it’s your pet theory at work — especially without any proof — that’s going to tell the reading public something about you as a source. And that something is along the lines of this:

“Claims of panspermia from Wickramasinghe are a) to be expected at every turn, and b) very likely to be exaggerated, without basis, or outright incorrect, for a variety of reasons.”

It’s a little like the story of the boy who cried wolf, but with a twist. Instead of being outright ignored, the way, say Giorgio Tsoukalos and his “ancient astronaut hypothesis” buddies are, in the scientific and skeptical communities he’s met, time and again, with analysis. Sure, if you’ve heard one Wickramasinghe explanation you’ve probably heard them all, and that is going to naturally colour your opinion every time he makes the same claim again, but it’s never taken outright for granted that he’s wrong, or that his reasoning is shoddy. Even though, every time so far, he or it has been.

So while the fact of someone being wrong (or their reasoning being poor) over and over doesn’t necessarily mean they’re wrong this time, it’s just a matter of due diligence to take it into account when looking into their most recent claims.

Ad hominem reasoning fails not because it takes the source into account, but because it stops there and goes no further. So to keep our integrity intact, we just need to use “oh it’s him (or her) again” as a starting point, and not an ending point. That is, if someone cries wolf panspermia again and again, while you’re less and less likely to believe them, it’s probably still worth checking.

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Richard Ford Burley is a doctoral candidate in English at Boston College, where he’s writing about remix culture and the processes that generate texts in the Middle Ages and on the internet. In his spare time he writes about science and skepticism (and panspermia) here at This Week In Tomorrow.