# Propositional Logic Symposia - [5] - Truth Functional Lingo, Syntax, and Calculations

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3. » Propositional Logic Symposia - [5] - Truth Functional Lingo, Syntax, and Calculations

Sun 8 Jun, 2008 02:02 pm
Thinking back on it, I could have incorporated this in the last symposium, but some things need to be cleared up before getting into complex truth tables and proofs.

Logical LanguageSentence Variables.
A sentence variables is basically any letter A to Z. So the variables in the sentence (A&B) --> C are A,B, and C. It is important to remember that any of these variables and any grouping of these variables is a sentence in truth functional logic. So A, B, A&B, AvB, A-->B, A<-->B are all sentences as well.

Sentence Operators
Sentence operators are the connectives; &, v, -->, <-->. (i.e., conjunction, disjunction, conditional, and bi-conditional).

Parenthetical Devices
The grouping devices used to isolate compound sentences from other sentences, such as; ( ), [ ], { }.

So now that you understand the essential elements of logical language, the formation of the sentence s you wish to translate must be well formed in order to be conveyed coherently.

Well Formed Formulas
This is essential in translating and proofs. You have to make sure you symbolize and simplify the translations you do before you get into the proof because if the formula you want to evaluate is not formed correctly, you will not be able to accurately translate is, find its truth value, etc.

There just a few things to remember.

TOO MANY BRACKETS!!!
Say you have this sentence; { [ (A&B) ] }
It seems obvious that those two outer parentheses (i.e. { } and [ ] ) can be dropped, because they are merely extra weight and mean nothing in the overall sentence. But you can also drop the ( ) as well because the only sentence is A & B. However, if the sentence was { [ ~(A & B) ] }, then you can drop the two outer brackets, but must leave the inner parentheses because of the tilde.

WTF!!! Deformed Well Formed Formulas!
The grammar within logic has to be very precise, and when translating sentences to logic may show one how good or bad their English composition is.

It is important to keep in mind that;

1.Variables cannot be side by side. Example: BN & H
2.Negations cannot be placed between two variables. Example: H ~N
3.Two connectives cannot be next to each other. Example: E &v H
4.Parentheses must be used. Example: A & N v C
5.Variables cannot float outside a parentheses. Example: N (D & B)

RECAP!!!!

Letters are variables, connectives are operators, Parentheses are parentheses! Also, to make a coherent formula and subsequent proof, you have to have a well formed formula, which is grammatically correct!

These things are the fundamental elements in the logical meta-language. It is important to cover these things now because you will need to know them in the complex truth tables. But before going into complex truth tables, let's look at basic truth table calculations which I think is pretty fun.

CALCULATING TRUTH VALUES
Best way to show this is to dive right into a sample problem from the very beginning. Suppose you had the sentence "Alan is playing and Bob is not."

Step 0. Fist and foremost before you begin, you have to identify the main operator that will determine the truth value of your sentence. This is obviously the conjunction symbol. It is not the negation because it is not a connective.
Step 1. Translate the problem correctly. Remember that if you are off in your translation, you will not be able to get a legitimate truth value.
Step 2. For now, we are supposing the truth values of our sentence. There is another way to do this, but it is much later in logic. Suffice to say that you need to provide the truth values yourself. But if you were looking for all the possible ways to see if the sentence could be true, do a basic truth possibilities matrix on the side and then input those possibilities into your calculation depending on how many variables you have. But for now, we understand that A is true and B is false.
Step 3. Replace the variables with the truth values you set in place.
Step 4.Step 5. This is where you memory comes in. YOU MUST REMEMBER THE RULES OF THE CONNECTIVES!!! So, the rule of the main operator ( & ) is "a conjunct is true if both conjuncts are true." Since the negation in step 4 turned the value of B to T, both conjunct are true, thus the sentence is true.

NOW FOR SOMETHING HARDER!

Now suppose this already translated sentence; ~ (~A --> ~B) v ~C.

Step 1. The sentence is already translated.
Step 2. Suppose the truth values or do a truth probability matrix calculate the possible truth values.
Step 3. Replace the variables with the truth values.
Step 4. In order to make things easier, do the truth values in an ordered way to keep track of your progress and serve as a back-up point in case you mess up on something.
Step 5. Next ordered step in the process
Step 6. Now that you have two truth values within a parentheses, you need to refer to the conditional rule, which states a conditional is false only when the antecedent is true and the consequent false. Since both variables of the conditional are not false, the conditional is true.
Step 7. Next order step in the process
Step 8. Refer to the disjunction rule to solve, which states that a disjunction is false only when both disjuncts are false.

That's basically what you have to know before we get into complex truth table analyses.

AGAIN, IF YOU HAVE ANY QUESTIONS, LET ME KNOW AND I'LL BE HAPPY TO ANSWER THEM.

ALSO, IF ANYONE WANT SAMPLE PROBLEMS, I CAN DRAW SOME UP FOR YOU OR CORRECT YOUR OWN.

Arjen

Sun 8 Jun, 2008 02:06 pm
@VideCorSpoon,
It is my understanding that no brackets are needed whatsoever, but I am not 100% sure anymore. I think there is always an order in which to work out the connectives. Do you by any chance have an example of where brackets are needed?

VideCorSpoon

Sun 8 Jun, 2008 02:24 pm
@Arjen,
That's a really good point that I didn't clarify.

Say you have the compound sentence; A & B.

You don't need any parentheses because if you include them in the simple statement A & B, you are implying that there are other operators and variables to consider. So in the example { [ ( A & B ) ] }, all of the parentheses and brackets are redundant because there is nothing to go in-between those divisions.

Now say you have the compound statement; (A & B) v C

You need those parentheses there to individuate the compound sentence "A & B" from "v C" to satisfy logical grammar. Now if you had all those brackets; { [ (A & B) v C ] }, you don't need either bracket because they are redundant.

Now say you have the compounded statement; [ (A & B) v C ] --> D

The brackets are necessary because that left antecedent has to be completely solved before the conditional can be solved. The brackets in this instance are necessary. But if we added another bracket; { [ (A & B) v C) --> D }, we wouldn't need it because it is redundant.

Now say you had the compounded statement { [ (A & B) v C ] --> D } <--> E

Then you would need all the parentheses and brackets.

Connectives

To work out connectives, you have to first identify the main connective of a problem. In the the case of { [ (A & B) v C ] --> D } <--> E for example, it is v (disjunction) because of the brackets which tell us that it has to hashed out before moving to the right bicondtional "E."

In A & B, the main connective is &
In (A & B) v C, the main connective is v
In [ (A & B) v C ] --> D, the main connective is -->
In { [ (A & B) v C ] --> D } <--> E , the main connective is <-->

VideCorSpoon

Sun 8 Jun, 2008 08:31 pm
@VideCorSpoon,

Main Connectives

1.A & B
2.(A v B) v (L & W)
3.B --> (L & N)
4.[ (B & ~A) --> (M -->L) ] --> Q

1.A & B
2.(A v B) v(L & W)
3.B --> (L & N)
4.[ (B & ~A) --> (M -->L) ] -->Q

Truth Value Calculations
(suppose A,B,C,D are true and E,F,G,H are false

1.A --> (B & C)
2.(A & B) & (C & D)
3.{ [ (A & B) --> (C v D) ] v (E & F) } v (G & H)

1. A --> (B & C)
a.T --> (T & T)
b.T --> T
c.T

2.(A & B) & (C & D)
a.(T &T) & (T & T)
b.T & T
c.T

3.{ [ (A & B) --> (C v D) ] v (E & F) } v (G & H)
a.{ [ (T & T) --> (T v T) ] v (F & F) } v (F & F)
b.{ [ T --> (T v T) ] v (F & F) } v (F & F)
c.[ ( T--> T ) v (F & F) ] v (F & F)
d.[ T v (F & F) ] v (F & F)
e.( T v F ) v (F & F)
f.T v (F & F)
g.T v F
h.T

Arjen

Mon 9 Jun, 2008 03:06 pm
@VideCorSpoon,
You are right. The brackets are just not always needed. The sequence of important connectives depends on the syntax. Funny how such basic things can slip the mind when not confronted with it.

Holiday20310401

Wed 22 Oct, 2008 07:44 pm
@Arjen,
Is there a sort of BEDMASS (if you are familiar with that trick) for placing brackets. I think the only hard part for me is understanding the prioritizing of connectives relative to other connectives, and I see a pattern from the sentences, but its fuzzy.

I am also starting to get confused with the idea of propositional logic and what it is intended for. It seems very limited. I mean, just because something is not false, doesn't make it true, right? But that's what you say in the symposium lesson here.

VideCorSpoon

Wed 22 Oct, 2008 08:15 pm
@Holiday20310401,
Yup! Generally it goes (), [], {}. It all depends on how many compound sentences you have. This was form another post in the symposia. Though the topic is more about the need for the brackets, look at the different separators as more complex formulas are made.

Say you have the compound sentence; A & B.

You don't need any parentheses because if you include them in the simple statement A & B, you are implying that there are other operators and variables to consider. So in the example { [ ( A & B ) ] }, all of the parentheses and brackets are redundant because there is nothing to go in-between those divisions.

Now say you have the compound statement; (A & B) v C

You need those parentheses there to individuate the compound sentence "A & B" from "v C" to satisfy logical grammar. Now if you had all those brackets; { [ (A & B) v C ] }, you don't need either bracket because they are redundant.

Now say you have the compounded statement; [ (A & B) v C ] --> D

The brackets are necessary because that left antecedent has to be completely solved before the conditional can be solved. The brackets in this instance are necessary. But if we added another bracket; { [ (A & B) v C) --> D }, we wouldn't need it because it is redundant.

Now say you had the compounded statement { [ (A & B) v C ] --> D } <--> E

Then you would need all the parentheses and brackets.

Holiday20310401

Wed 22 Oct, 2008 08:58 pm
@VideCorSpoon,
{ [ (A & B) v C ] --> D }

So if I imply truth variables to the variables, and simplify it to just T or F, what am I proving when I get to T and F. What do they mean? Is T just saying that the truth value is true, and F saying that the truth value is false?

Will I always be able to simplify it to just T or F, or by virtue of being able to do so, I am proving that the function is valid. And if the function cannot be simplified it is invalid?

Lemme just try this one and see if I'm on right track here.
I will say that A=T B=T C=F D=F
Therefore, (is there a bb script for therefore where you get those 3 dots by any chance) { [ (T & T) v F ] --> F }
Since T & T = T I get... { [ T v F ] --> F }
Since T v F = T I get... { T --> F }
Therefore I get F, since T--> F = F. Did I do ok? Basically, I have to look at the charts you gave us at the beginning to discern with what connectives do I arrive at what truth value, or intuitively grasp it:rolleyes:.

VideCorSpoon

Thu 23 Oct, 2008 07:56 am
@Holiday20310401,

Zacrates

Fri 19 Jun, 2009 12:41 pm
@VideCorSpoon,
how do you use this in a real world argument??? sorry if i missed something or sound stupid.... so far i get all of it, and i got everything right, i just dont understand how to use it in an argument, or to prove something with it... I will read on and recoment if i still dont get it.

VideCorSpoon

Sat 20 Jun, 2009 11:16 am
@Zacrates,
Take this proposition;

If Alan runs to the bank, then Bob runs to the bank. If Bob runs to the bank, then Charlie runs to the bank. However, Charlie is not running to the bank.

What could you conclude from this ridiculous proposition? You could actually get a narrowed, logical inference from what had transpired in that paragraph. But what is important is to address "how do you use this (logic) in a real world argument?"

The best way I can think of why we would want to use formal logic as a tool in deciphering a real world argument is in terms of the LSAT (law school admissions test). The reason why I am going to talk about the LSAT and logic is to show how you can use logic in a real world argument, or at least a test question, so you can see how it helps you decipher an argument faster and more precisely than others who do not know the system.

In the LSAT, you have a series of questions (ironically, one section is called "logic games") in which you have to answer in a certain amount of time. It is widely acknowledged that everyone would get a perfect 180 (highest score possible) if they had enough time. Unfortunately, they give you about a minute for each question. How would you be able to solve the paragraph above in less than a minute? To be honest, I would take at least five minutes trying to grasp the statement and figure out what the whole deal of it is. But utilizing formal logic, I could solve the paragraph in a few seconds, adding those remaining seconds to other questions.

The sample paragraph is almost exactly like a question you would see on the LSAT and is an extremely soft ball question. The LSAC (law school admissions council) purposefully puts these questions in to give you more time for other, harder questions. In a way, questions like these are a test in themselves because the LSAC know that a knowledge of formal logic makes you much better suited for legal studies, and gear the test accordingly by giving those who know the system more time to do other questions. This would also (on a side note) would explain why the average of the LSAT is a 140 something (literally a C average).

But anyway, the question. Presented with the question on the piece of paper in front of you, I would use logic in a real world argument like this;

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