# Symbolic Logic Help

• Apr 6th 2010, 06:59 AM
johnnyboy1
Symbolic Logic Help
Hey guys,

I've been studying this book "Introduction to Logic" and have come across 3 tough questions I was hoping I could get explained to me as I have no clue where to start. They deal with the rules of inference and the rules of replacement.

1. (D⊃F) • (P⊃N)
2. D v P
3. (D ⊃ ~N) • (P⊃ ~F)
4. (D ⊃ F) / F ≡ N

(I started off here simplifying the 1st line, but that's as far as I got)
---------------------------

1. (Q v ~T) v S
2. ~Q v (T • ~Q) / T ⊃ S

-------------------------

and

1. (C v B) ⊃ (T • L)
2. ~C ⊃ (K ⊃ ~K)
3. ~T / ~K

I'd honestly appreciate any help whatsoever, even some hints. Thanks again.
• Apr 6th 2010, 08:57 AM
PiperAlpha167
Quote:

Originally Posted by johnnyboy1
Hey guys,

I've been studying this book "Introduction to Logic" and have come across 3 tough questions I was hoping I could get explained to me as I have no clue where to start. They deal with the rules of inference and the rules of replacement.

1. (D⊃F) • (P⊃N)
2. D v P
3. (D ⊃ ~N) • (P⊃ ~F)
4. (D ⊃ F) / F ≡ N

(I started off here simplifying the 1st line, but that's as far as I got)
---------------------------

1. (Q v ~T) v S
2. ~Q v (T • ~Q) / T ⊃ S

-------------------------

and

1. (C v B) ⊃ (T • L)
2. ~C ⊃ (K ⊃ ~K)
3. ~T / ~K

I'd honestly appreciate any help whatsoever, even some hints. Thanks again.

The name of the author might be helpful.

Anyway, looking at the first sequent, the fourth premiss looks redundant to me.
On top of that it's not a valid sequent.
Consider the valuation: v(F) = T, v(N) = F, v(D) = T, v(P) = F.

However, replace the conclusion by its negation; then the sequent is valid and a derivation is pretty straight forward.
The big steps would be two applicatons of the rule of inference formulation of complex constructive dilemma.
At that point, you'll have (F V N) & (~F V ~N). (Think definition of exclusive disjunction.) The rest is busy work.
• Apr 6th 2010, 11:08 AM
emakarov
Quote:

I've been studying this book "Introduction to Logic" and have come across 3 tough questions
So what do the questions ask to do?

Quote:

4. (D ⊃ F) / F ≡ N
At first I thought, Interesting, they invented a new propositional connective: division. I am wondering what its truth table, axioms or inference rules are. Then I realized that this is a task to derive one formula from several others:

1. (D⊃F) • (P⊃N)
2. D v P
3. (D ⊃ ~N) • (P⊃ ~F)
4. (D ⊃ F)
----------------------------
F ≡ N

Also, I assume that the bullet • denotes conjunction, right?

Are you supposed to use truth tables or inference rules?
• Apr 6th 2010, 12:38 PM
johnnyboy1
all 18 inference rules. yep, • is a conjunction. (i.e Mary went shopping and Gary played video games = M • G)

I got the last 2 questions btw:

(Q v ~T) v S, ~Q v (T • ~Q) : T ⊃ S

1. (Q v ~T) v S; Premise
2. ~Q v (T • ~Q); Premise
3. Q v (~T v S); 1 Association
4. Q v (T ⊃ S); 3 Implication
5. (~Q v T) • (~Q v ~Q); 2 Distribution
6. ~Q v T; 5 Simplification
7. ~Q v ~Q; 5 Simplification
8. ~Q; 7 Tautology
9. T ⊃ S; 4, 8 Disjunctive Syllogism

(C v B) -> (T & L), ~C -> (K -> ~K), ~T : ~K

1. (C v B) -> (T & L) : Premise
2. ~C -> (K -> ~K) : Premise
3. ~T : Premise
4. ~T v ~L : 3 Addition
5. ~(T & L) : 4 DeMorgan's Theorem
6. ~(C v B): 1, 5 Modus Tollens
7. ~C & ~B : 6 DeMorgan's Theorem
8. ~C : 7 Simplification
9. ~B : 7 Simplification
10. K -> ~K : 2, 8 Modus Ponens
11. ~K v ~K : 10 Implication
12. ~K : 11 Tautology

Thanks for the help emakarov and Piper
• Apr 8th 2010, 04:48 PM
xalk
Quote:

Originally Posted by johnnyboy1
all 18 inference rules. yep, • is a conjunction. (i.e Mary went shopping and Gary played video games = M • G)

I got the last 2 questions btw:

(Q v ~T) v S, ~Q v (T • ~Q) : T ⊃ S

1. (Q v ~T) v S; Premise
2. ~Q v (T • ~Q); Premise
3. Q v (~T v S); 1 Association
4. Q v (T ⊃ S); 3 Implication
5. (~Q v T) • (~Q v ~Q); 2 Distribution
6. ~Q v T; 5 Simplification
7. ~Q v ~Q; 5 Simplification
8. ~Q; 7 Tautology
9. T ⊃ S; 4, 8 Disjunctive Syllogism

This proof is wrong.In line (4) you cannot conclude :Q v (T ⊃ S) by Implication.

You could conclude that if (~T v S) was on its own and not within another formula

T
he rule for implication is : (A=>B)<=>(not(A) v B)

Which is your A and which is your B in this case??

The right proof is:

1. (Q v ~T) v S; Premise
2. ~Q v (T • ~Q); Premise
3. Q v (~T v S); 1 Association
4. ~Q⊃ ( ~Tv S); 3 Implication
5. (~Q v T) • (~Q v ~Q); 2 Distribution
6. ~Q v T; 5 Simplification
7. ~Q v ~Q; 5 Simplification
8. ~Q; 7 Tautology
9. ~T v S 4,8 Modus Ponens
10. T ⊃ S ; 9 Implication
• Apr 9th 2010, 01:25 AM
PiperAlpha167
Quote:

Originally Posted by johnnyboy1
all 18 inference rules. yep, • is a conjunction. (i.e Mary went shopping and Gary played video games = M • G)

I got the last 2 questions btw:

(Q v ~T) v S, ~Q v (T • ~Q) : T ⊃ S

1. (Q v ~T) v S; Premise
2. ~Q v (T • ~Q); Premise
3. Q v (~T v S); 1 Association
4. Q v (T ⊃ S); 3 Implication
5. (~Q v T) • (~Q v ~Q); 2 Distribution
6. ~Q v T; 5 Simplification
7. ~Q v ~Q; 5 Simplification
8. ~Q; 7 Tautology
9. T ⊃ S; 4, 8 Disjunctive Syllogism

(C v B) -> (T & L), ~C -> (K -> ~K), ~T : ~K

1. (C v B) -> (T & L) : Premise
2. ~C -> (K -> ~K) : Premise
3. ~T : Premise
4. ~T v ~L : 3 Addition
5. ~(T & L) : 4 DeMorgan's Theorem
6. ~(C v B): 1, 5 Modus Tollens
7. ~C & ~B : 6 DeMorgan's Theorem
8. ~C : 7 Simplification
9. ~B : 7 Simplification
10. K -> ~K : 2, 8 Modus Ponens
11. ~K v ~K : 10 Implication
12. ~K : 11 Tautology

Thanks for the help emakarov and Piper

You're welcome.

Both of your derivations look fine to me.

(A minor point regarding the second derivation is that line 9 is superfluous.
There's no need to break out ~B by Simp. since it's not used subsequently.)

On the other hand, regarding the first derivation, I think it appropriate to emphasize the fact that you've
correctly applied a rule of replacement (what you've called "implication") to a subwff of the wff at line 3 to obtain the wff at line 4.