Propositional natural deduction
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1 Propositional natural deduction COMP2600 / COMP6260 Dirk Pattinson Australian National University Semester 2, 2016
2 Major proof techniques 1 / 25 Three major styles of proof in logic and mathematics Model based computation: truth tables for propositional logic Algebraic proof: simplification rules e.g. De Morgan s Deductive reasoning: rules of inference Natural deduction is one example Other examples: tableau calculi, resolution, Hilbert calculi
3 Example proof using truth tables 2 / 25 Statement to be proved: (p (q r)) ((p q) r)) For all 8 (= 2 3 ) possibilities of p,q,r, calculate truth value of the statement p q r q r p (q r) p q (p q) r (p (q r)) ((p q) r))
4 Example proof using truth tables 2 / 25 Statement to be proved: (p (q r)) ((p q) r)) For all 8 (= 2 3 ) possibilities of p,q,r, calculate truth value of the statement p q r q r p (q r) p q (p q) r (p (q r)) ((p q) r)) T T T T T T T T
5 Example proof using truth tables 2 / 25 Statement to be proved: (p (q r)) ((p q) r)) For all 8 (= 2 3 ) possibilities of p,q,r, calculate truth value of the statement p q r q r p (q r) p q (p q) r (p (q r)) ((p q) r)) T T T T T T T T T T F T T T T T
6 Example proof using truth tables 2 / 25 Statement to be proved: (p (q r)) ((p q) r)) For all 8 (= 2 3 ) possibilities of p,q,r, calculate truth value of the statement p q r q r p (q r) p q (p q) r (p (q r)) ((p q) r)) T T T T T T T T T T F T T T T T T F T T T F T T
7 Example proof using truth tables 2 / 25 Statement to be proved: (p (q r)) ((p q) r)) For all 8 (= 2 3 ) possibilities of p,q,r, calculate truth value of the statement p q r q r p (q r) p q (p q) r (p (q r)) ((p q) r)) T T T T T T T T T T F T T T T T T F T T T F T T T F F F F F F T
8 Example proof using truth tables 2 / 25 Statement to be proved: (p (q r)) ((p q) r)) For all 8 (= 2 3 ) possibilities of p,q,r, calculate truth value of the statement p q r q r p (q r) p q (p q) r (p (q r)) ((p q) r)) T T T T T T T T T T F T T T T T T F T T T F T T T F F F F F F T F T T T F F T T
9 Example proof using truth tables 2 / 25 Statement to be proved: (p (q r)) ((p q) r)) For all 8 (= 2 3 ) possibilities of p,q,r, calculate truth value of the statement p q r q r p (q r) p q (p q) r (p (q r)) ((p q) r)) T T T T T T T T T T F T T T T T T F T T T F T T T F F F F F F T F T T T F F T T F T F T F F F T
10 Example proof using truth tables 2 / 25 Statement to be proved: (p (q r)) ((p q) r)) For all 8 (= 2 3 ) possibilities of p,q,r, calculate truth value of the statement p q r q r p (q r) p q (p q) r (p (q r)) ((p q) r)) T T T T T T T T T T F T T T T T T F T T T F T T T F F F F F F T F T T T F F T T F T F T F F F T F F T T F F T T
11 Example proof using truth tables 2 / 25 Statement to be proved: (p (q r)) ((p q) r)) For all 8 (= 2 3 ) possibilities of p,q,r, calculate truth value of the statement p q r q r p (q r) p q (p q) r (p (q r)) ((p q) r)) T T T T T T T T T T F T T T T T T F T T T F T T T F F F F F F T F T T T F F T T F T F T F F F T F F T T F F T T F F F F F F F T
12 The notion of a deductive proof 3 / 25 A proof is a sequence of steps. Each step is either: an axiom or an assumption; or a statement which follows from previous steps via a valid rule of inference. Natural deduction: for each connective, there is an introduction and an elimination rule rules are formal, but resemble natural, i.e., human reasoning
13 Example of a natural deduction proof 4 / 25 Statement to be proved: (p (q r)) ((q s) p) 1 p (q r) Assumption 2 p -E, 1 3 (q s) p -I, 2 4 (p (q r)) ((q s) p) -I, 1 3
14 Conjunction rules 5 / 25 -I (and introduction) p q p q
15 Conjunction rules 5 / 25 -I (and introduction) p q -E (and elimination) p q p q p p q q
16 Example 6 / 25 Commutativity of conjunction (derived rule) p q q p 1 p q 2 p -E, 1 3 q -E, 1 4 q p -I, 2, 3
17 Implication rules 7 / 25 -I (implication introduction) [ p ]. q p q This notation means that by assuming p, you can prove q. p is then discharged - no longer an assumption.
18 Implication rules 7 / 25 -I (implication introduction) [ p ]. q p q This notation means that by assuming p, you can prove q. p is then discharged - no longer an assumption. -E (implication elimination) p p q q
19 Example - transitivity of implication (derived rule) 8 / 25 We prove p q q r p r
20 Example - transitivity of implication (derived rule) 8 / 25 We prove p q q r p r 1 p q
21 Example - transitivity of implication (derived rule) 8 / 25 We prove p q q r p r 1 p q 2 q r
22 Example - transitivity of implication (derived rule) 8 / 25 We prove p q q r p r 1 p q 2 q r 3 p
23 Example - transitivity of implication (derived rule) 8 / 25 We prove p q q r p r 1 p q 2 q r 3 p 4 q -E, 1, 3
24 Example - transitivity of implication (derived rule) 8 / 25 We prove p q q r p r 1 p q 2 q r 3 p 4 q -E, 1, 3 5 r -E, 2, 4
25 Example - transitivity of implication (derived rule) 8 / 25 We prove p q q r p r 1 p q 2 q r 3 p 4 q -E, 1, 3 5 r -E, 2, 4 6 p r -I, 3 5
26 Example - transitivity of implication (derived rule) 8 / 25 We prove p q q r p r 1 p q 2 q r 3 p 4 q -E, 1, 3 5 r -E, 2, 4 6 p r -I, 3 5 Lines 1 and 2 are the assumptions given, you may use these throughout the proof Line 3 is an assumption made within the proof, you may use it only within its scope (lines 3 to 5)
27 Notation: justification of a step 9 / 25
28 Notation: justification of a step 9 / 25 1 p q
29 Notation: justification of a step 9 / 25 1 p q 2 p
30 Notation: justification of a step 9 / 25 1 p q 2 p 3 q -E, 1, 2
31 Notation: justification of a step 9 / 25 1 p q 2 p 3 q -E, 1, 2 4 p q -I, 2 3
32 Notation: justification of a step 9 / 25 1 p q 2 p 3 q -E, 1, 2 4 p q -I, 2 3 This is a rather silly proof, we succeed in proving what we started with. But it illustrates the meaning of the line number notation: -E,1,2 means that rule -E proves line 3 from lines 1 and 2 -I,2-3 means rule -I proves line 4 from the fact that we could assume line 2 and (using that assumption) prove line 3
33 Rules involving assumptions 10 / 25
34 Rules involving assumptions 10 / 25 1 p q
35 Rules involving assumptions 10 / 25 1 p q 2 q r
36 Rules involving assumptions 10 / 25 1 p q 2 q r 3 p
37 Rules involving assumptions 10 / 25 1 p q 2 q r 3 p 4 q -E, 1, 3
38 Rules involving assumptions 10 / 25 1 p q 2 q r 3 p 4 q -E, 1, 3 5 r -E, 2, 4
39 Rules involving assumptions 10 / 25 1 p q 2 q r 3 p 4 q -E, 1, 3 5 r -E, 2, 4 6 q r WRONG -I, 4, 5
40 Rules involving assumptions 10 / 25 1 p q 2 q r 3 p 4 q -E, 1, 3 5 r -E, 2, 4 6 q r WRONG -I, 4, 5 If a statement is inside the scope of an assumption, then it depends on that assumption. Given p q and q r, we then assumed p and proved q r, but q r depends on p. Indentation and vertical lines indicate scoping Similar to scoping in program code: eg lines 3 to 5 are a a method, and p is a local variable to that method.
41 Useless assumptions 11 / 25 You can assume anything, but it might not be useful.
42 Useless assumptions 11 / 25 You can assume anything, but it might not be useful. 1 p q
43 Useless assumptions 11 / 25 You can assume anything, but it might not be useful. 1 p q 2 q -E, 1
44 Useless assumptions 11 / 25 You can assume anything, but it might not be useful. 1 p q 2 q -E, 1 3 (p q) q -I, 1 2
45 Useless assumptions 11 / 25 You can assume anything, but it might not be useful. 1 p q 2 q -E, 1 3 (p q) q -I, p You are a giraffe 2 You are a giraffe -E, 1 3 p You are a giraffe You are a giraffe -I, 1 2
46 Disjunction rules 12 / 25 -I (or introduction) p p q p q p
47 Disjunction rules 12 / 25 -I (or introduction) p p q p q p -E (or elimination) [p] [q].. p q r r r
48 -E template 13 / 25
49 -E template 13 / 25 1 p q
50 -E template 13 / 25 1 p q 2 p
51 -E template 13 / 25 1 p q 2 p..
52 -E template 13 / 25 1 p q 2 p.. a r
53 -E template 13 / 25 1 p q 2 p.. a b r q
54 -E template 13 / 25 1 p q 2 p.. a b r q..
55 -E template 13 / 25 1 p q 2 p.. a b r q.. c r
56 -E template 13 / 25 1 p q 2 p.. a b r q.. c r d r -E, 1, 2 a, b c
57 Example: commutativity of disjunction (derived rule) 14 / 25 p q q p
58 Example: commutativity of disjunction (derived rule) 14 / 25 1 p q p q q p
59 Example: commutativity of disjunction (derived rule) 14 / 25 p q q p 1 p q 2 p
60 Example: commutativity of disjunction (derived rule) 14 / 25 p q q p 1 p q 2 p 3 q p -I, 2
61 Example: commutativity of disjunction (derived rule) 14 / 25 p q q p 1 p q 2 p 3 q p -I, 2 4 q
62 Example: commutativity of disjunction (derived rule) 14 / 25 p q q p 1 p q 2 p 3 q p -I, 2 4 q 5 q p -I, 4
63 Example: commutativity of disjunction (derived rule) 14 / 25 p q q p 1 p q 2 p 3 q p -I, 2 4 q 5 q p -I, 4 6 q p -E, 1, 2 3, 4 5
64 Negation rules 15 / 25 Idea: assume the opposite of what you want to prove and find a contradiction so your assumption must have been wrong -I (not introduction) [p]. q q p
65 Negation rules 15 / 25 Idea: assume the opposite of what you want to prove and find a contradiction so your assumption must have been wrong -I (not introduction) -E (not elimination) (eliminates from an assumption) [p]. q q p [ p]. q q p
66 Example: double negation introduction (derived rule) 16 / 25 p p It is raining It is not the case that is is not raining
67 Example: double negation introduction (derived rule) 16 / 25 p p It is raining It is not the case that is is not raining
68 Example: double negation introduction (derived rule) 16 / 25 p p It is raining It is not the case that is is not raining 1 p
69 Example: double negation introduction (derived rule) 16 / 25 p p It is raining It is not the case that is is not raining 1 p 2 p
70 Example: double negation introduction (derived rule) 16 / 25 p p It is raining It is not the case that is is not raining 1 p 2 p 3 p p -I, 1, 2 4 p I, 2 3
71 Example: contradiction elimination (derived rule) 17 / 25 p p q
72 Example: contradiction elimination (derived rule) 17 / 25 p p q
73 Example: contradiction elimination (derived rule) 17 / 25 p p q 1 p p
74 Example: contradiction elimination (derived rule) 17 / 25 p p q 1 p p 2 q
75 Example: contradiction elimination (derived rule) 17 / 25 p p q 1 p p 2 q 3 p p R, 1
76 Example: contradiction elimination (derived rule) 17 / 25 p p q 1 p p 2 q 3 p p R, 1 4 q E, 2 3
77 Example: double negation elimination (derived rule) 18 / 25 p p
78 Example: double negation elimination (derived rule) 18 / 25 p p
79 Example: double negation elimination (derived rule) 18 / 25 1 p p p
80 Example: double negation elimination (derived rule) 18 / 25 p p 1 p 2 p
81 Example: double negation elimination (derived rule) 18 / 25 p p 1 p 2 p 3 p p -I, 1, 2 4 p E, 2 3
82 Equivalence 19 / 25 p q means p is true if and only if q is true We can make the definition p q (p q) (q p) which would naturally give us these rules introduction rule: p q q p elimination rules: p q p q p q p q q p
83 Equivalence Rules 20 / 25 Alternatively we can get rules which don t involve the symbol -I ( introduction) [p] [q] -E ( elimination).. q p p q p q p p q q Note the similarities to the -I and -E rules q p
84 Which rule to use next? 21 / 25 Guided by the form of your goal, and what you already have proved form ie, look at the connective:,,, always can consider using -E (not elimination) rule to prove p q, -I (or introduction) may not work p p q q p q p may not be necessarily true, q may not be necessarily true
85 To prove p q, sometimes you need to do this: 22 / 25 1 Using -E, assume (p q) (hoping to prove some contradiction) 2 When is (p q) true? When both p and q false! 3 From (p q) how to prove p? (next slide) 4 Having proved both p and q, prove some further contradiction Exercise: prove p q p q
86 Not-or elimination (derived rule) 23 / 25 (p q) p
87 Not-or elimination (derived rule) 23 / 25 (p q) p
88 Not-or elimination (derived rule) 23 / 25 (p q) p 1 (p q)
89 Not-or elimination (derived rule) 23 / 25 (p q) p 1 (p q) 2 p
90 Not-or elimination (derived rule) 23 / 25 (p q) p 1 (p q) 2 p 3 p q -I, 2
91 Not-or elimination (derived rule) 23 / 25 (p q) p 1 (p q) 2 p 3 p q -I, 2 4 (p q) (p q) -I, 1, 3
92 Not-or elimination (derived rule) 23 / 25 (p q) p 1 (p q) 2 p 3 p q -I, 2 4 (p q) (p q) -I, 1, 3 5 p I, 2 4
93 Proving a contrapositive rule 24 / 25 In the same way, whenever you can prove any q then you can prove p p q
94 Proving a contrapositive rule 24 / 25 In the same way, whenever you can prove any q then you can prove p p q
95 Proving a contrapositive rule 24 / 25 In the same way, whenever you can prove any q then you can prove p p q 1 q
96 Proving a contrapositive rule 24 / 25 In the same way, whenever you can prove any q then you can prove p p q 1 q 2 p
97 Proving a contrapositive rule 24 / 25 In the same way, whenever you can prove any q then you can prove p p q 1 q 2 p 3 q your proof of q from p
98 Proving a contrapositive rule 24 / 25 In the same way, whenever you can prove any q then you can prove p p q 1 q 2 p 3 q your proof of q from p 4 q q -I, 1, 3
99 Proving a contrapositive rule 24 / 25 In the same way, whenever you can prove any q then you can prove p p q 1 q 2 p 3 q your proof of q from p 4 q q -I, 1, 3 5 p I, 2 4
100 Law of the excluded middle (derived) 25 / 25 p p Everything must either be or not be. Russell
101 Law of the excluded middle (derived) 25 / 25 p p Everything must either be or not be. Russell
102 Law of the excluded middle (derived) 25 / 25 p p Everything must either be or not be. Russell 1 (p p)
103 Law of the excluded middle (derived) 25 / 25 p p Everything must either be or not be. Russell 1 (p p) 2 p -E (previous slide), 1
104 Law of the excluded middle (derived) 25 / 25 p p Everything must either be or not be. Russell 1 (p p) 2 p -E (previous slide), 1 3 p -E (previous slide), 1
105 Law of the excluded middle (derived) 25 / 25 p p Everything must either be or not be. Russell 1 (p p) 2 p -E (previous slide), 1 3 p -E (previous slide), 1 4 p p -I, 2, 3 5 p p E, 1 4
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