I’ve said before that I think common sense can be a dangerous term. My argument then was as follows:
If someone tells you that x is the case, based on common sense, then I think they mean that x being the case is self-evidently true. But if it is self-evidently true, then pointing that out is redundant. At best what they’re saying is, “Look, maybe it’s not obvious right now, but if you just consider it for a moment, you’ll see that it is. And if you can’t see that, you must lack common sense.” It’s that last implication that’s really dangerous, because if x is not self-evidently true, then calling it common sense is designed to discourage you from questioning something dubious.
If you’re interested in an example of how the term is abused, I recently found this. It’s a site maintained by a man called Harper who has some very odd ideas indeed about language. He believes, for example, that Latin was invented by Italian speakers as a kind of secret shorthand, that French is descended from English, which is not descended from Old English (you can read more here and here). On his website he claims, however, to offer “new theories” that are “not weird” and which are “either closely evidenced or chock-full of common sense”. There’s something troubling about that. First I have doubts about how “closely evidenced” his theories really are. Second, while it’s good and indeed necessary to apply one’s trained intuitions (a very charitable interpretation of what he means by common sense) to the evidence, our intuitions alone—especially untrained ones—are a very poor substitute for real evidence and reasoned appraisal of it.
Harper is right that we shouldn’t just accept wholesale what people with letters after their name tell us, but he’s wrong to suggest that common sense is a very reliable means of distinguishing when we should or shouldn’t trust what we’re told. Real evidence and our intuitions often lead in different directions. If we’re trying to understand the world, then we need to be prepared for the truth to run contrary to our common-sense assumptions. We’re much more likely to accept something unquestioningly if it accords with our biases—if it seems, in other words, like common sense—and encouraging people to follow that route is to lead them into error.