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This question has perplexed me since the ninth grade. You're in a room with only two doors. One door leads to death, the other to life. Each door has a guard. One guard always lies, the other always tells the truth. You don't know which door is which, and you don't know which guard is which. How do you get out by asking one guard just one question? Is it possible? My math teacher never told us. I think this one is much harder than that stupid "sailors at the motel" riddle. --Thomas Bronaugh, Moore, Oklahoma

Hah. This was so easy I didn't even need to apply my brain to the problem. I let my tonsils figure it out. Answer at the bottom of the column.

THE LAST WORD ON EXCEPTIONS

I hate to have to correct Cecil Adams, but the business about "the exception proves the rule" in the latest Straight Dope [October 25] seems way wide of the mark. The proverb's meaning must be expounded not in the context of natural or psychological law but of civil law. Alan Bliss, in A Dictionary of Words and Phrases in Current English, has the following to say about the origin of this phrase: "Exceptio probat regulam [Lat.], the exception proves the rule. A legal maxim of which the complete text is: exceptio probat [or (con)firmat] regulam in casibus non exceptis--'the fact that certain exceptions are made (in a legal document) confirms that the rule is valid in all other cases.'"

The application is this. Suppose a law is stated in such a way as to include an exception, e.g., "Parking is prohibited on this street from 7 AM to 7 PM, Sundays and holidays excepted." The explicit mention of the exception means that no other exceptions are to be inferred. Thus we should take the Latin verb probare in the maxim to have the sense of "to increase the force of." --Hugh Miller, Chicago

You're quite right. In fact, much as it wounds me to admit it, yours may well be the original sense of this proverb (although my interpretation, namely that the exception tests the rule, has a long and respectable history of its own). Trouble is, your example stinks. We need something that better conveys the import of this ancient maxim. I have just the thing--an illustration from the Roman orator Cicero, sometimes cited as the source of the legal doctrine in question.

Cicero was defending one Bilbo. (No, you silly thing, not that Bilbo.) Bilbo was a non-Roman who was accused of having been illegally granted Roman citizenship. The prosecutor argued that treaties with some non-Roman peoples explicitly prohibited them from becoming Roman citizens. The treaty with Bilbo's homeboys had no such clause, but the prosecutor suggested one should be inferred.

Nonsense, said Cicero. "Quod si exceptio facit ne liceat, ubi non sit exceptum . . ." Oops, I keep forgetting how rusty folks are on subjunctives. What he said was if you prohibit something in certain cases, you imply that the rest of the time it's permitted. To put it another way, the explicit statement of an exception proves that a rule to the contrary prevails otherwise.

You can see where an argument like this would come in handy in traffic court. What's more, it's basically what Kyle Gann was arguing in his letter in the October 25 column, although he mixed in another much looser interpretation, namely that exceptions call attention to the rule, which obscured matters a bit. Nonetheless he did not deserve the abuse I dished out and I apologize.

ANSWER

Schoolmarm's version: Ask either guard, "If I were to ask you if your door was the death door, would you say yes?" If he says yes, that's the death door. If not, it's not.

Real-world version: Ask, "How'd you like to look like you got run over by a truck if you don't show me which is the right door?" Lacking in the intellectual niceties, sure, but it gets the job done.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.

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