Take the 2-minute tour ×
English Language & Usage Stack Exchange is a question and answer site for linguists, etymologists, and serious English language enthusiasts. It's 100% free, no registration required.

Is the conclusion true here?

Either Peter did not know it or Sarah did know it. But Peter did know it. Therefore, Sarah did know it.

share|improve this question
    
This won't get you anywhere. Language is not logic. Unlike symbols in formal logic that are, by design, unambiguous, words in natural languages are, by design, ambiguous. In fact a word can, and often does, mean the exact opposite of itself. That includes "either... or". If you ask me, your conclusion is neither correct nor incorrect — it is not a conclusion in the first place, it's a non sequitur. –  RegDwigнt Jun 26 '13 at 9:07
add comment

2 Answers

You mean "Therefore, Sarah did not know it"?

That would be the conclusion that fits with both logic and English. "Either...or" is exclusive-or in English. There is usually confusion when people use or alone; it's unclear whether it's exclusive-or or inclusive-or. In English, or alone is usually exclusive, whilst in logic it is strictly inclusive. People sometimes use and/or when they want to express inclusive-or.

For more information see this excellent Yahoo answer.

share|improve this answer
    
Thanks for that input. But the question is as written. The question is either/ or. The way I'm reading it is: Since Peter did not Not know it -as opposed to did not know it-Sarah did know it. Therefore the given conclusion of Sarah did know it. –  user40011 Jun 26 '13 at 8:23
add comment

There are four cases:

Case 1: Sarah and Peter both know it.
Case 2: Sarah and Peter both do not know it.
Case 3: Sarah knows it but Peter does not.
Case 4: Peter knows it but Sarah does not.

We are given "Either Peter did not know it or Sarah did know it". This rules out case 4.

We are now told "But Peter did know it". This rules out cases 2 and 3.

Since 2, 3, and 4 are ruled out, case 1 must be correct. So the conclusion is correct, Sarah and Peter must both know it, hence Sarah must know it.

(The above assumes the "or" is intended to be inclusive. The final result is the same if you understand it to be exclusive, though the reasoning is slightly different -- case 3 is ruled out earlier.)

share|improve this answer
    
Thanks. That was very useful –  user40011 Jun 26 '13 at 9:04
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.