Here’s a logical fallacy I’ve been coming across (and making) lately.
“Non semper, ergo numquam” means “Not always, therefore never”.
In response to pointing out some cause of an event, or a criticism of an idea, a person finds a counter example. That can’t be true because of a particular example when it is not true.
It is a kind of quantification fallacy, a false conversion, but from my research I could not find a name for it.
It is a fallacy because most of the time we make general statements not with the purpose of suggesting they apply in all cases, just that they are general enough to remark on. To undermine such a general statement it is not enough to respond with a counter-example.
So “natural selection drives evolution”… “here’s a situation where natural selection doesn’t drive evolution.”
Or “religion promotes supernatural nonsense”… “I’m religious and I don’t believe in the supernatural.”
Or “marijuana is not a particularly harmful drug”… “But my cousin did weed, and ended up in a psych hospital.”
But still, the counter example is a tempting rebuttal, and a strong rhetorical trick. I’ve found myself often trying to justify the counter-example: trying to reject it or cast doubt on it. Which is a tacit acceptance that the generalization was supposed to apply in all cases. It inverts the discussion, so a reasonable statement gets turned into an unreasonable exclusive statement, and the person is forced to defend that unreasonable position. The correct response is more like “so?” But even that can feel like a weakening of the argument. Though I fight it, I instinctively love black-and-white certainties. And even when I am intending to suggest a matter is only a very dark shade of grey, it can be galling to be reminded it is not entirely black.
It is particularly galling when faced with someone with a tendency to commit the opposite fallacy: “aliquando ergo semper” (sometimes therefore always) – someone who will take the valid counter example as evidence that the matter is entirely white. Who’ll conclude that, if natural selection is not the mechanism behind all evolution then it can be entirely discounted, or that the presence of any historical material in the bible means the bible is historically accurate. In those cases an unambiguous counter claim would be preferable, for the same of rhetoric. But this fallacy is always there for the over-eager.