Propositional Logic and English
Date: 03/04/2003 at 17:39:23 From: Jason Subject: Propositional logic I wish to ask for some assistance in clearing up a couple of issues that arose in a class in propositional logic. Given the propositions: P: The sun shines Q: The wind blows R: The rain falls S: The temperature rises We were asked to write in words various compound propositions, one of which was… Q Xor R (Q Exclusive-Or R) Would it be correct to say, "Either the wind blows or the rain falls"? This seems correct to me. The 'either' word emphasizes the fact that 'the wind blows' OR 'the rain falls' but not both (Xor). Even though the 'either' word strengthens the sentence, wouldn't anyone who said 'The wind blows or the rain falls' still be implying that only one or the other of the propositions is true and not both? in which case, the question is what, in an English sentence, is the difference between Q Xor R and Q Inclusive-Or R? Another problem arose with the proposition (P Inclusive-Or S) AND R in which the answer I came up with was "The sun shines or the temperature rises and the rain falls" This also seems fine except that the symbolic compound proposition allows the possibility of 'The sun shines' and 'the temperature rises' together (Inclusive-Or), which seems to contradict what was mentioned in the previous problem! Also, how do you differentiate, in English, (P Inclusive-Or S) AND R from P Inclusive-Or (S AND R) "The temperature rises and the rain falls or the sun shines"? These compound propositions obviously produce different truth tables but both seem to produce the same meaning in English, just in a different order. It is only now that I can see just how ambiguous the English language can be! I hope you can help and perhaps give a few more examples to demonstrate. Thanks a lot!
Date: 03/08/2003 at 14:08:26 From: Doctor Achilles Subject: Re: Propositional logic Hi Jason, Thanks for writing to Dr. Math. For general help in logic, you can refer to the Dr. Math crash course in symbolic logic: http://mathforum.org/dr.math/faq/symbolic_logic.html You are absolutely right that the English language is ambiguous when it comes to logical expressions. As if the examples you gave weren't enough, we also have a page in our archives describing some further ambiguities from inclusive vs. exclusive ORs. Exclusive or Inclusive Disjunction? http://mathforum.org/library/drmath/view/55692.html It's a wonder that we ever communicate anything at all! The page cited above argues that for many English sentences it is almost impossible to take the sentence by itself and decide what the propositional equivalent is, that the entire context is needed. But enough of that digression, let's look at the questions you have. >Q Xor R (Q Exclusive-Or R) >Would it be correct to say. >"Either the wind blows or the rain falls"? Although it is possible to have an "either...or" sentence in English that actually represents an inclusive-or, the default interpretation of "either...or" is exclusive-or. So I would say that "Either the wind blows or the rain falls" is the best English translation of your sentence. You can, of course, get more precise with "Either the wind blows or the rain falls, but both do not happen simultaneously," but I think that this sentence (while more precise) is awkward English and not something one would ever expect to hear outside of a logic class, so in my opinion your translation is the best. >(P Inclusive-Or S) AND R >In which the answer I came up with was >"The sun shines or the temperature rises and the rain falls" This problem is a bit trickier. First, the issue of inclusive- vs. exclusive-or: the omission of the word "either" means that the default interpretation of this sentence is inclusive-or, but it is somewhat ambiguous. You can get more concrete by using the word "and/or" which is used with some frequency in written English, but very rarely in spoken English, e.g. "The sun shines and/or the temperature rises and the rain falls." Once again, although this is more precise, is it also very inelegant English, and so I think that you're probably just as well off sticking with the simple "or" that you have. The more difficult problem is the ambiguity between (P INC-OR S) AND R and P INC-OR (S AND R) In the first sentence, the main connective is "AND"; it joins the subsentence (P INC-OR S) to the subsentence R In the second sentence, the main connective is "INC-OR"; it joins the subsentence P to the subsentence (S AND R) For a review of what "main connectives" are, see Main Connectives in a Proof http://mathforum.org/library/drmath/view/55623.html I'm not sure I can give a definitive set of rules to deal with this problem. However, in my experience with English it seems to me that in most cases the first connective you use is interpreted as the main connective. In cases where it is impossible to do this, you can emphasize the main connective with a comma in written English or a pause in spoken English. The sentence >"The sun shines or the temperature rises and the rain falls" is ambiguous, but if forced to decide, I would say probably means P INC-OR (S AND R) because the "or" comes first. In this case, I think you have to change the order of the sentence. There are two logically equivalent sentences which are easier to understand (in my opinion). Instead of: (P INC-OR S) AND R try translating the logically equivalent: (P AND R) INC-OR (S AND R) or just: R AND (P INC-OR S) The first would translate to something like: "The sun shines and the rain falls, or the temperature rises and the rain falls." This sentence is pretty clear, and the comma (or pause in spoken English) will emphasize that the inc-or is the main connective. However, I don't like this sentence very much because it is awkward, and the double use of "the rain falls" is inelegant. My preferred translation is to start with R AND (P INC-OR S) and turn it into: "The rain falls, and the sun shines or the temperature rises." This may be a little more ambiguous than the option above, but it is much more elegant. Also, the comma (or pause) before the and will put enough emphasis on it to make it clear that the and is the main connective. A sentence in logic can get much more complicated. For example, you can have something like: (P AND Q) INC-OR (S AND R) This you could translate into "The sun shines and the wind blows, or the temperature rises and the rain falls." Worse still is: [(P INC-OR Q) AND (S INC-OR R)] INC-OR T [Where T means "there is a tornado"] Once you get into sentences which contain more than two layers of connectives like this, you pretty much have to break it up into two English sentences, e.g. "The sun shines or the wind blows, and the temperature rises or the rain falls. Or, there is a tornado." The worst case for translation, in my opinion, is if you mix inclusive- and exclusive-or's in the same sentence. For example: (P AND Q) INC-OR (S X-OR R) One possible translation for this is "The sun shines and the wind blows, or either the temperature rises or the rain falls." I think this is inelegant and difficult to understand because of the two or's in the same clause. The logically equivalent "Either the temperature rises or the rain falls, or the sun shines and the wind blows" is a little better, but it is still difficult to make it clear that the second or is inclusive. Finally, (P X-OR Q) INC-OR (S X-OR R) Is really hard. "Either the temperature rises or the rain falls, or either the sun shines or the wind blows" sounds just awful and can't really be helped without breaking the translation into multiple sentences. "Either the temperature rises or the rain falls. Or, either the sun shines or the wind blows" is an improvement, which works for written English. For spoken English, however, I'm stumped on this one. I guess the best thing to do is just have a long pause before and after the "Or" at the start of the second sentence. You can get even trickier, and probably find sentences that can't be translated into elegant English and just plain require the use of terms like "exclusive-or" in their "English" translation. Hope this helps. If you have other questions or you'd like to talk about this some more, please write back. - Doctor Achilles, The Math Forum http://mathforum.org/dr.math/
Date: 03/12/2003 at 15:10:12 From: Jason Subject: Thank you (Propositional logic) Thanks for the help. The pages you recommended to look at within your archives were very useful. Jason
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