### The Logic Forum Discussion Area

Logic
Author
Comment
Re: Need help with Truth-Functional Logic please!

Hi Fred, I've taken the liberty of adding your e-mail to my mailing list.

Re: Need help with Truth-Functional Logic please!

Thanks for your intervention, Jack. I have not checked your work, but you seem to know what you are doing.

Re: Need help with Truth-Functional Logic please!

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.

Re: Need help with Truth-Functional Logic please!

Hi Jack,

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.

Re: Need help with Truth-Functional Logic please!

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!

Re: Need help with Truth-Functional Logic please!

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.

Re: Need help with Truth-Functional Logic please!

Avi,

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!

Re: Need help with Truth-Functional Logic please!

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.)

Re: Need help with Truth-Functional Logic please!

Fred,

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.

And is it not also necessary to additionally note that (¬S → ¬O) is true in addition to what is presented in number 2?

¬S => ¬O translates to "If Daisy does NOT prefer salad to dessert then James is NOT outraged". But this is saying more than your original statement, so I would say no, and in fact it would be wrong to include it. But see Avi's comment above regarding "it does not follow", and my reply to him below.

Of course there are often equivalent translations into logic for any given proposition, but they should all produce the same truth table. If you generate truth tables for your three statements you'll see that they are not equivalent.

There are many logic calculators online which will do the tedious work for you. A nice one is here:

https://www.erpelstolz.at/gateway/TruthTable.html

Re: Need help with Truth-Functional Logic please!

Avi,

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.

Yes, I'm currently studying FL. I guess my point was that modern logic has no way of "symbolically" representing a nonsequitur ("it does not follow"). The result of negating a conditional is

¬(p => q) = p & ¬q

But this doesn't have the same meaning as "q does not follow from p", or "if p, not-then q". It's one of the problems with a purely "truth-functional" logic as you clearly explain in Chapter 24.3.

Re: Need help with Truth-Functional Logic please!

Thanks Jack.

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.

Thanks :)