peeve no. 254 is "argument from common sense"

"Common sense is the collection of prejudices acquired by age eighteen." -Albert Einstein (link)

In the midst of an argument, the most satisfying retort is exposing the opponent's reasoning as a fallacy, such as one of these. But the most frustrating reasoning to deflate is reasoning that's not even wrong: its truth value is meaningless, mu, zero (and not the way C conditionals judge zero). The "argument from common sense" is in this category of fallacy. When someone advances an idea on the basis of, or defended by, what he/she calls common sense, no recourse is available. Possibly one could gain some satisfaction by responding with "I find your abundance of faith in unexamined assumptions disturbing", but that still wouldn't be a real counterargument.

Real counterarguments to the argument from common sense would be meaningless, because the argument from common sense is meaningless, because common sense is meaningless in an objective debate. I don't mean that the concept of "common sense" is meaningless, but that its very usefulness is its function as a catchall term for implicit knowledge. It doesn't have a fixed definition. Without sharp definitions, debates can endlessly spin in confusion purely from misunderstanding of what each participant is communicating. (Not incidentally, this is why the legal documents of a society under the rule of law are careful to define all important nouns. If you use the GPL, you really should know exactly what a "derivative work" is.)

Moreover, a bare argument from common sense exudes sheer ignorance of observer bias and disregard for rigorously evaluating truth via proof--how much of common sense is the result of investigation, as opposed to culture and mindless conformity? As I've heard some people say, with tongue-in-cheek, "common sense isn't common". A subset of common sense knowledge for one person, or even an entire group, might not be a subset of common sense knowledge for another person, or even another entire group. Is it common sense to use global variables sparingly, if at all? Is it common sense to indent code in a consistent manner? Is it common sense to change the default/root password? Is it common sense to finally cease using <font> tags, for Bob's sake? Maybe for some people, those procedures are so blindingly obvious that using the description "common sense" is pretty much an insult to the words. For other people, perhaps not...

The argument from common sense is not an argument. If trying to justify a statement by referring to common sense, find some other way. Look closer at what "common sense" really means in that specific case. At least make your assumptions explicit, so everyone knows you aren't using common sense as a logical hand-wave (no need to look here, it's common sense!). Common sense has done a lot for you. Don't push it into being something it isn't.