Sure, it can be a flimsy support for a generalization, but it’s not an entirely useless phrase. It makes a point that something may be generally true if the only exceptions that one can come up with are particularly out of the ordinary. (I’m not sure if my explanation is working here.) Certainly, outside of formal logic, most rules have exceptions, and so if you make a statement, and only one counterexample can be found, or if the few counterexamples found can be explained away, then you’ve given some thin bolstering to your argument. Certainly not enough to hold up in court, but I don’t think the expression’s exactly meaningless.
The only way I can see this phrase make sense in the present day is if Person A made a rule and told Person B, Person B wants to find the flaw in the rule, after much thought Person B comes up with an exception, and Person A exclaims “That’s the exception that proves the rule” Person B is left bewildered, and then Person A shows person B how the exception he has come up with is invalid, and that since that is the only exception he can come up with, it actually proves his rule once he negates it. Sounds Sherlock Holmesish to me. Something that Holmes would say to Watson, or Velma would say to shaggy
here is a rough example:
Person A: The butler always kills the mistress
Person B <after much thought>: What about case number 47, didnt the maid kill the mistress?
Person A: Hmm…case 47…oh well there you go! That is the exception that proves the rule!
Person B: I’m sorry, but I don’t understand your logic
Person A: In case 47 if i recall correctly, The mistress was shot in the back by the made correct?
Person B: Well yes of course
Person A: Well upon further examination and questioning I discovered that The maid and the butler were having an affair. The butler, upset about his pay wanted to get rid of the mistress who was determining his payroll. The butler confided in the maid his desire to kill the mistress. The butler then bought a Smith & Wesson revolver, gave it to the maid, and instructed her to do the dirty work for him. So you see, as the gun used the bullet to commit murder, the maid used the gun, and the butler used the maid, thereby he being the one who committed the murder!
Person B: I dare say you are brilliant ol’ chap! It certainly is the exception that proves the rule!
<They go off for tea and crumpets>
No.
Every rule has an exception. Except this one.
I read several books by Achebe for a class I took in college, and to me his style of storytelling defines the statement “the exception the proves the rule”. In all of his stories, the main characters “buck” their tribe or society, or otherwise make decisions or have personality traits that are just not normal. So the fact that Bob the nigerian tribesman was angry and beat his wives and loved to fight actually helped to prove how calm and unaggressive the society was, because it contrasted so sharply with everyone around him.
This sort of exception/societal contrast thing was all throughout these books. It ranged from belief in magic, to temprament, to obeying the gods, to theft, to pride and ego and just about anything else you can think of. In every case the main characters were exceptions that helped prove the rule.