Propositional Logic Symposia - General Logic Questions

[Glemkat]

Hello! I want to first say that these have been very helpful in trying to clarify Logic. I have a question that does not seem to have been posted (or I haven't been able to find it).

What about subproofs? I have been trying to do a few subproofs in my assignments and I am having trouble doing those. I have listed an example of one of the problems. I have tried to go through many options but I can't seem to come up with how to do these types of problems effectively. We are just at Negation Rules and can't use Taut Con, Ana Con, etc. We can only use Intro and Elim (all: &, V, Contradiction). If someone can give me pointers on how to do these, I would be able to decipher the answer on my own. I really appreciate any help. I feel like I just keep hitting a brick wall with this class.

Premises:

1. Dodec(e)
2. ~Small(e)
3. ~Dodec(e) V Dodec(f) V Small(e)

Conclusion:

Dodec(f)

I used three subproofs since I have three premises. With the last one, I was able to end up with the conclusion fairly easily. With the first and second one, I ended up just getting to contradiction which I am not sure what that even means.

Any help would be greatly appreciated!

[VideCorSpoon]

Please use this thread to post your questions about propositional logic.

Note to Glemkat: this is not about you, I just wanted to add a general question thread for use in the future and your question was a perfect starting question. Sorry about the move.

[VideCorSpoon]

Hi Glemkat!

I am not sure what you are referring to when you say sub-proofs. I have heard of sub-formulas from predicate logic, but I think it may just be a specific rule that you are using that goes by a different name than what I know of. But I am sure I can help you with your question.

Could you elaborate some more on the issue you are having?

[Glemkat]

Hello VideCorSpoon,

Thank you for your prompt reply and your note about posting general questions.

A subproof in my book is a proof within a proof. I am attaching a screenshot of the problem that I have been working on (at least I hope I did). Know that the proof is incorrect. I was just going through and seeing what I can prove to get to the conclusion.

I would assume that the subproof is the same as what you stated in your previous message about predicate logic. What I would do is take one of the premises and form another proof underneath the premises. I hope that I am making sense. I am thoroughly confused....

glemkat

[VideCorSpoon]

No problem at all. First I should note is that we are in a sense doing mathematics, but in different languages. Case in point, we are both doing propositional logic, but under different understood systems, so it may take a little bit to understand the system you are using since I am used to a different type of symbolic logic. What are the rules (full names) you can use at this point?

In the mean time, when you are talking about “sub-proofs,” I am interpreting this as inference proofs. I use the Herrick system primarily, so the two inference proofs are Indirect proof and conditional proof. Indirect proofs states that to prove P, assume ~P and derive a contradiction and then assert P. A direct proof on the other hand requires a conditional, assuming the antecedent, derive the conditional, and assert the conditional. Yours may be different though, so there needs to be more clarification.

[Glemkat]

Sorry for the delay in getting back to you on the clarification.

I looked up indirect proofs and conditional proofs online. The conditional proof looks more like what I am currently doing. I think that I did find in a search where you talked about conditional proof here. I did read that but I am not understanding where to begin on more challenging problems. This problem would have three conditional proofs because of three premises. That is what I was thinking anyway. I am not sure. I hope that helps. Thanks for being patient with me.

[VideCorSpoon]

What is the exact name of your classification of logic? The type of propositional logic that I am using here is set propositional logic. This also goes by other names like sentential logic and SD logic, depending on whose system you are using. Something tells me you are using an arithmetic grounded logic like Boolean or recursion because from what you have said so far, you use a much different set of rules. Then again, you may be doing formal proofs judging from the example you sent. So I am thinking it is an issue in proof structure and translation.

In the propositional logic system I use for example, the basis for conditional proofs is determined by the possibilities of inference, not the basis of premises. You can of course have more than one conditional proof in the same proof (i.e. nested proofs) but it is in a much different format than yours. Same rules, different configuration.

Here is an example of a proof in my system with a single inferred conditional.

http://i44.tinypic.com/ipd3sm.jpg

I'm trying to piece together your system and mine so that we can move on.

[facepuncher]

if the comma is the determinate in the statement, how would something like this be symbolised?

it's cold, but it's not windy or foggy

i was thinking (C ~(W v F)). am i close at all? is it appropriate to have a basic proposition followed by a negated compound?

C: it's cold W: it's windy F: it's foggy by the way

HALP

[IMO]

What is the value of translating English into logic?

How to symbolize a compound sentence----------------------------------------------------------------------------
Now take the compound sentence; “John is a funny guy and Mary is a funny girl.”

1.First, isolate the two sentences you see in the compound sentence; “John is a funny guy and Mary is a funny girl.”
2.Now take what we know about connectives from symposium 3 and translate the connective “and” into the connective “&.”
3.Now we can translate the compound sentence into; J & M.

[VideCorSpoon]

On a general level, the main reason one would want to translate English into logic is so that that English sentence (compound or otherwise) will fit into the formal propositional logic (in this case) system. You need to in order to utilize the system. But I suppose you are probably asking “why logic?”

Suppose I said this to you; “If Alan is at home, the Bob is at home. If Bob is at home, then Charlie is at home. Thus, if Alan is at home, then Charlie is at home.” In the broader range of critical thinking, this may seem like a complex scenario, and thinking about it may take someone a little bit of time to comprehend the “value” of the compound statement. But if you utilize a formal logic system, it makes perfect sense and just a glance infers a truth functional rule, which is that of a hypothetical syllogism.

A hypothetical syllogism breaks down like this (at least to ma at any rate.)

Compound syntactical structure;
“If Alan is at home, the Bob is at home. If Bob is at home, then Charlie is at home. Thus, if Alan is at home, then Charlie is at home.”

Breakdown;
If Alan is at home, the Bob is at home. = A-->B
If Bob is at home, then Charlie is at home. = B -->C
Thus, if Alan is at home, then Charlie is at home. = |- A-->C

Propositional formula for compound statement;
A-->B,B-->C, |- A-->C

From this equation, I can infer, deduce, etc. in a variety of symmetric (or even asymmetric) ways to approach an argument. When you get into logic, translating the sentenced you read for an argument into logical syntax turns out to be a lot easier than just looking at the superficial statement.

The reason for translating English into logic is in a sense a short cut for deductive argumentation and so on. You could very well do without the finer points made in propositional, predicate, or any other type of logic, but then you end up coming around full circle again. You would come up with shortcuts (deductions) which would necessitate the very system one would try to avoid. There are many reasons to study logic, such as an increased ability to identify logical structures in arguments given to you, a more refined way to evaluate arguments and the ability to find out what , and just the simple ability to construct and present good arguments for yourself. (Herrick regurgitation) And this is just a broad rationalization for the system, because this is not even factoring in truth-functional implications, necessary or sufficient conditions, and all the other stuff inherent in the knowledge of the system. As to the “value” of translating English to logic, what better way to find the ”value” than within a truth functional system like logic. But in my mind, the real value in all of this comes from the part when logic is then translated into English. When you get to that point, you sweat half as much in a task twice as hard as the normal deductive processes we would normally use.