Avi, you wrote:
Thanks for your intervention, Jack. I have not checked your work, but you seem to know what you are doing.
Fred, another word of advice if I may. It is better for you to present your proposed solution to the problem and then have that corrected by others, than to ask others to do the thinking for you. One learns best from one's errors (if any), than from ready-made work by others.
Thanks again for your message. I think we have been taught to use disjunction if a sentence is in the form of 'unless A, B'. But it is important to note that we can also offer alternative disambiguations if they exist. I've got this (O ∨ ¬(S ∧ Y) ∧ L) as what I think is right, because it can be paraphrased in English as 'James is outraged unless it is not the case that Daisy prefers salad to desert and Ed hates Yorkshire puddings, and that Ed laughs.
Does this make sense? Thanks.
Thank you Avi. I've noted my idea of the solution below, but I'm not 100% sure about it. If you can think of any disambiguations for this statement that would be a real help. No worries if you don't have time, I really appreciate it nonetheless!
Hi Jack and Avi,
Thanks for your messages. So far, I have come up with three possible disambiguations of this statement: James is outraged unless Daisy doesn’t prefer salad to dessert, Ed doesn’t hate Yorkshire puddings, and Ed doesn’t laugh.
The first one uses 'unless' to imply a disjunction, whereas the other two use a conditional. If you could comment on which ones you think most accurately represent the sentence in TFL I would be very thankful.
1. (O ∨ ¬(S ∧ Y ∧ L))
2. ((S → O) ∧ ¬Y ∧ ¬L)
3. ((S → O) ∧ (¬S → ¬O) ∧ ¬Y ∧ ¬L)
Jack, you're right, but not entirely. Inattention on my part, though.
"I'll go out unless it's raining" =
If it does not rain, then I will go out, AND
If it rains, not-then I will go out (i.e. in that event I may not go out).
"The plant will die unless you water it" =
If you don't water the plant, then it will die, AND
If you water the plant, not-then it will die (i.e. in that event it may not die)
A unless B =
If not B, then A, AND
If B, not-then A (i.e. if B, possibly not A)
The reason for this is to shut off the possibility of both "If not B, then A" and "if B, then A" being true, in which case A would be true categorically. The negative side ensures we have a conditional statement.
The negative side in both cases has a not-then (= a then-possibly-not) consequent.
I wrongly stated it as a then (or then-necessarily) statement.
My apology. Thanks for correcting me.
Thanks for the clarification. As far as I'm aware modern propositional logic doesn't have a way of stating that a proposition B does NOT follow from another proposition A, like your innovation of "if A, not-then B", and in my opinion is all the poorer for it!
Hi Jack, thanks for the reply. Whilst I do agree with you, do you think that the other representations are also acceptable? Could this sentence not be seen to use a disjunction rather than a conditional? And is it not also necessary to additionally note that (¬S → ¬O) is true in addition to what is presented in number 2? Thanks
Jack, logical conditioning is dealt in great detail in Future Logic,part III.
That said, the form If/not-then is nothing very new really. In ordinary discourse, we say commonly things like "if so and so, it does not follow that such and such". If X, not-then Y is stated in modern logic as simply the negation of if X then Y, i.e. as not(X->Y), i.e. X does not imply Y.
Keep in mind that for every form we need a counter form (though there are very rare exceptions, as I recall).
(Jack, please help Fred if you can. As I said, I am in the middle of a difficult bit of writing. It is difficult for me to turn my attention elsewhere. Many thanks.)
Hi Fred, I've taken the liberty of adding your e-mail to my mailing list.
Glad that Jack is answering your questions.
Statements 1 and 3 don't really work.
1. O or ¬(S & Y & L)
By De Morgan's rule, the right disjunct is ¬S or ¬Y or ¬L, so you have O or ¬S or ¬Y or ¬L.
Rearranging, this is (¬S or O) or ¬Y or ¬L. Now, (¬S or O) is equivalent to S => O, which is what we need, and the remaining propositions ¬Y and ¬L should be combined conjunctively with this conditional according to the original statement, but they are instead combined disjunctively.
I just wanted to remind you of the first few questions I asked if thats ok. I noticed you used conditionals to represent the double-turnstile symbol in the truth table. In my textbook, the double turnstile is used to symbolise tautological validity between arbitrary sentences of TFL. It is not a symbol of TFL, but rather a symbol of augmented English used to describe arbitrary sentences of TFL. The conditional is a symbol of TFL, unlike the double-turnstile. Also, the letters in the questions are used to symbolise any arbitrary sentence of TFL, not specific ones.
Based on this information, would this change any of your answers?
I have put the questions I am struggling with below. Sorry to bring this up again, I just wanted to check.
Question: Are the following claims about tautological validity true or false?*
*If you could explain your position on each question that would be really useful.
1. If A ⊨ B, then B ⊨ A.
2. If A ⊨ C, then A ∧ ¬C ⊨ C.
3. If A ∨ B ⊨ C ∧ D, then A ⊨ C.
I also want to say how grateful I am for your help. I am working through this textbook by myself and your explanations have really helped me over the last week.