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An Introduction to Classical Logic (propositional and Predicate Logic)
Lecture Notes for ISA 780/SWE 623 by Duminda Wijesekera ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Propositional and Predicate Logic
Propositional Logic The study of statements and their connectivity structure. Predicate Logic The study of individuals and their properties. Study syntax and semantics for both. Propositional logic more abstract and hence less detailed than predicate logic. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Propositional Logic: Syntax
A collection of atomic propositional symbols. Say A = { ai : 0 < i }. A special atom _|_ for contradiction A collection of logical connectives. (and) ^, (or) v, ( not ) , (implies) => Inductively define propositions as: If X,Y are propositions, so are :– X ^ Y, X v Y, X => Y, X. Examples: a1^a2, (a =>a2)v(a3^( a4)) are propositions. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Propositional Logic: Semantics
A model M of a propositional language consists of a collection of atoms, say B = { bi : 0 < i }, where _|_ is excluded from B, and a partial mapping M from A = { ai : 0 < i } to B = { bi : 0 < i }. If M(ai) e B, we say that ai is true in M. We write “ai is true in M” as M |= ai. (Read M satisfies ai). ╞ is referred to as the satisfaction relation. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Propositional Semantics: Continued
Extend M, and therefore the satisfaction relation to all propositions using the following inductive definition: M ╞ X ^ Y iff M ╞ X and M ╞ Y. M ╞ X v Y iff M ╞ X or M ╞ Y. M ╞ X => Y if M ╞ X then M ╞ Y. M ╞ X, if it is not the case that M ╞ X. Notice usage of truth tables ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Propositional Logic: Example
B = { a1, a3} where M given as M(a1) = a1 and M(a2) = a2 has the following properties. M ╞ a1 M ╞ a1 ^ a3 M ╞ a2 M ╞ a2 => a4 M does not satisfy the following propositions. M ╞ a4 M ╞ a1 => a4 ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Propositional Logic: Proofs
What formulas hold in all models ? I.e. can we check if a given proposition is true in all models without going through all possible models? Need proofs to answer this question. We use Natural Deduction proofs. Recommended: Read Ch 2 of Logic and Computation by L.C. Paulson. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Natural Deduction for Prop. Logic
Proofs are trees of formulae made by applying inference rules. An inference rule is of the form: A1 …… An B Here A1 ….. An are said to be premises (or antecedents) of the rule, and B is said to be the conclusion (consequent) of the rule. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Natural Deduction for Prop. Logic
Hence a proofs is a trees whose Root is the theorem to be proved, Branches are rules, and Leaves are the assumptions (axioms) of the proof. Example A1 A2 A3 C1 C2 Assumptions B B2 Applications of rules D Theorem being proved There are introduction and elimination rules for each connective in Natural Deduction proof systems. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Rules for Conjunction Introduction Elimination A B A ^ B A ^ B A ^ B
ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Rules for Disjunction Introduction Elimination
A B A v B A v B Elimination [A] [B] A v B C C C [X] denotes discharged assumption X. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Rules for Implication Introduction Elimination (Modus Ponens) [A] B
A => B A ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Rules for Negation B interpreted as ( B => _|_). Hence we get the following rules from those of implication. Introduction Elimination [B] B B _|_ _|_ ________ B Special Contradiction Rule: B _|_ __________ B ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Propositional Proofs: Examples
Prove: ( A ^ B ) => (A v B) Notice: The outermost connective is =>. Hence, the last step of the proof must be an implication introduction. That means, we must assume ( A ^ B ) and prove (A v B), and then discharge the assumption by using => introduction rule. In order to prove (A v B) from ( A ^ B ), we must use v –introduction, and hence must prove either A or B from ( A ^ B ). This plan forms a skeleton of a proof. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Prop. Proof: Example Continued
Prove: ( A ^ B ) => (A v B) [A ^ B ] A ^ elimination A v B v introduction ( A ^ B ) => (A v B) => introduction Proofs are analyzed backwards, I.e. start unraveling the logical structure of the conclusion and work backwards to the assumptions. Draw out a plan based on your analysis and write down the formal proof. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Derived Rules These are rules derived from other rules. Example:
A ^ B B ^ A Here is the derivation: A ^ B B ^ A B A ^ elimination B ^ A ^ introduction ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Soundness and Completeness
A rule A1 …… An is said to be sound if for every B model in which all of A1 …… An are true, then so is B. I.e. if M ╞ A1 , …… , M |= An, then M ╞ B. A collection of rules are sound if all rules in the collection is sound. A collection of rules is complete if M ╞ A for all models M, then A is provable. I.e. there is a proof of A using the given set of rules. (Denoted |R-- A ) where R is the set of rules. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Predicate Logic Language to describe properties of individuals.
Thus, syntax is able to describe individuals, their properties (relationships) and functions. These are to be thought of as names of individuals, properties (relationships) and functions. Models are “incarnations” of these individuals, properties (relationships) and functions. More detailed than propositional logic. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Predicate Logic: Syntax
A collection of constants– say { ci : i >= 0 }. Constants are names for individuals. E.g.: 0, 1. Note: not all individuals in a model have names. A collection of variables– say { xi : i >= 0 }. Needed to generically refer to individuals. Think of them as standing in place of pronouns like it, she. A collection of function symbols- say { fi : i >= 0 }. May be of different arities, and may be typed. E.g.: +(x,y) A collection of predicate symbols- say { pi : i >= 0 }. May be of different arities. Encodes properties of individuals. E.g.: prime(x). ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Predicate Logic Recursive Definition of Terms
Every variable is a term. Every constant is a term. If fi is an n-ary function symbol and t1, .., tn are terms, then fi(t1, .., tn) is a term. We use {ti : i <=0 } for the collection of terms. Examples: f(x, g(2, y)) is a term, where f, g are function symbols and x, y are variables. +( x, *(3,y)) is a term in arithmetic usually written as x + (3*y) ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Recursive Definition of Formulas
If pi is an n-ary predicate symbol and t1, .., tn are terms, then pi(t1, .., tn ) is an atomic formula. If A and B are formulas, then so are: A ^ B, A v B, A, A => B. xi A(xi), xi A(xi), where xi is a variable. , are referred to as the universal and existential quantifier, respectively. A formula that does not have either quantifier is said to be a quantifier free. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Free and bound Variables
In x A(x), the variable x is said to be bound; meaning the name x plays no significant role. (compare with he, she, it) A variable x occurs bound in a formula if x or x is a part of it. More precisely, x occurs bound in: y A(y) or y A(y) if x and y are the same variable. A if x occurs bound in A. A ^ B, A v B, A => B if x occurs bound in either A or B. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Substitutions If A is a formula, t is a term and x is a variable, then A[t/x] is the formula obtained by substituting t for x in A. A[t1/x1, … tn/xn] is the formula resulting in simultaneously substituting x1, …xn by t1, …tn. Note: Simultaneous substitution Q(x,y)[x/y,y/x] yields Q(y,x) but iterated substitution Q(x,y)[x/y][y/x] yields Q(y,y). ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Substituting Terms for Variables
In A[t/x], the free variables of t stand the danger of becoming bound in A. Hence, need a precise definition. If x is y then y A(y) [x/y] is y A(y). If not let z be a fresh variable (I.e. not in t, x) then (y A(y) )[t/x] is z (A(z/y) [t/x]). Similar definition for y A(y). Examples: y (y = 1) [y/y] is y (y = 1). Here x is y and t is x. y (y+1 > x) [2y+x/x] is z ((z+1>x)[2y+x/x] I.e. z (z+1>2y+x). Here t is (2y+x). ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Substituting Terms Continued
( A )[t/x] is (A [t/x]) (A ^ B) [t/x] is (A[t/x] ^ B[t/x]) (A v B) [t/x] is (A[t/x] v B[t/x]) (A => B) [t/x] is (A[t/x] => B[t/x]) Pi(t1, .. tn) [t/x] is Pi(t1[t/x], .. tn[t/x]) for predicate symbol Pi. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Predicate Logic: Semantics
A model consists of A set (of individuals), say A = { ai : i >= 0 }. A set of total functions Fn = { fni : i >= 0 } on A. I.e. fni(aj) is some ak for every aj. A set of predicates Pr = { pri : i >= 0 } over A. Do not have to be total. Can have many arities. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Interpreting Syntax Mapping from Syntax to Semantics:
A mapping mCons : { ci : I >= 0 } to A={ai: i >= 0}. Need not be ONTO A. I.e. there could be unnamed individuals in the semantic domain. A mapping mFun : { fi : I >= 0 } to Fn={fni: i >= 0}. Need not be onto. I.e. there could be unnamed functions in the semantic domain. A mapping mPred: { pi : I >= 0 } to Pr={pri: i >= 0}. Need not be onto. I.e. there could be unnamed predicates in the semantic domain. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Interpreting Formulas: naming
We do not interpret formulas with free variables. In order to interpret quantified formulas, need to expand the syntax by adding a constant in the syntax for each unnamed individual in the model. I.e. for each ai for which there is no cj such that Fn(cj ) is ai, add a new constant Cai to the syntax. Now expand the definition of terms to include these new constants. Let newT = { Nti : i >= 0} be the collection of new terms so defined. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Interpreting Formulas
Let M be a model. We define M ╞ F for every quantified formula as follows. For every n-ary predicate symbol pi , and every sequence of new variable free terms Nt1, … Ntn define M ╞ pi(Nt1, … Ntn ) if and only if mPred(pi)(Nt1, … Ntn ). I.e. pi(Nt1, … Ntn ) is true in M if and only if its image under the map mPred holds with parameters Nt1, … Ntn . ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Interpreting Formulas: Continued
For every formula A , M ╞ y A(y) if and only if M ╞ A(Nti) for every Nti e newT. For every formula A , M ╞ y A(y) if and only if there is some Nti e newT satisfying M ╞ A(Nti). M ╞ A ^ B if M ╞ A and M ╞ B . M ╞ A v B if M ╞ A or M ╞ B. M ╞ A => B if when M ╞ A then M ╞ B. M ╞ A if it is not the case that M ╞ A. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Proof Rules for Predicate Logic
Proof rules of introduction and elimination of ^, v, =>, and . New rules required for introduction and elimination of and quantifiers. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Proof Rules for Introduction Elimination
A(x) provided x is not free in the x A(x) assumptions of A Elimination x A(x) A[t/x] ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Proof Rules for Introduction A[t/x] xA(x) Elimination
[A] provided x is not free xA(x) B in B nor in the B assumptions of B apart from A ISA 780/SWE 623Classical Logic Duminda Wijesekera
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An Example Proof Prove ((x A(x)) ^ B) => (x (A(x)^ B)) provided that x is not free in B. Plan: Since outer connective is =>, need to use => introduction at the last step. Hence can use (x A(x)) ^ B as an assumption for the steps above. Now in order to get x (A(x)^ B) using introduction, we need to get A[t/x] )^ B. Can use ^ elimination to (x A(x)) ^ B and obtain B Can use x elimination to get A[t/x]. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Example Proof x A(x) ^ B x A(x) ^ B x A(x) [A(t/x)] B A(t/x) ^ B
The other direction of the proof appears in the handout page 32. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Induction Rule [A(x)] A[0/x] A[x+1/x] A(x)
Proviso: x is not free in the assumptions of A[x+1/x] apart from A(x). ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Equality Reasoning Rules for equality
Reflexivity axiom: t = t. Symmetry rule: t = u . u = t Transitivity rule: s = t t = u . s = u Congruence laws for each function and predicate symbol, or substitution rules. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Equality Reasoning: Continued
Congruence Law for functions: t1 = u1 …. tn = un f(t1, …., tn) = f(u1, ….,un) Congruence Law for Predicates: p(t1, …., tn) p(u1, ….,un) Substitution Rule: t = u S[t/x] = S[u/x] ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Equality Reasoning: An Example
x f(x,x) = x f(g(z), g(z)) = g(z) p(f(g(z), g(z)) p(g(z)) p(f(g(z), g(z)) p(g(z)) ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Many Sorted Logic Sorts can be introduced to first-order logic
So what we learned is single-sorted first order-logic x: Integer [ y: Real divides(x,y) ] Integer and Real are two different sorts. x,y: Point z: Line [x ≠ y => isOn(x,y,z)] Point and Line are sorts ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Translating many sorted Logic to one-sorted logic
Create a one-ary predicate for each sort and write the statement as an implication. Many sorted: x: Integer [ y: Real divides(x,y) ] One Sorted: x y[isInteger(x) ^ isReal(y)=>divides(x,y)] x,y: Point z: Line [x ≠ y => isOn(x,y,z)] x,y z[ isPoint(x) ^isPoint(y) ^ x ≠ y => isLine(z) isOn(x,y,z)] ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Notations from Z In Z x f(x) is written as x .f(x)
Sometimes types are also used right after the quantifier, for example: x :opera · isComposer(Beethovan,x) x: Integer · [ y: Real . devides(x,y) ] ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Typed Logics Sorts are expected to be disjoint sets That implies:
Integers are NOT a subset of Real Numbers Introducing types are more complicated Requires that there be type constructors. Example: Integer Real Ordered pairs, say ORD Example x: ORD, z: Integer z: Real x=(y,z) Here ( , ) is a type constructor. ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Why is this an Issue? How to prove: (x,y) = (a,b) iff x=a and y=b
Similar problems occur with sets, lists etc. What is the problem? The equality theory has to be built. Sets create more problems: Set quantifiers are problematic Leads to FULL second order logic No complete Σ1 proof theory ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Using Logic As a Programming Language
Prolog Datalog Basic Idea: Use them as Rules Can specify search Raises many issues with semantics ISA 780/SWE 623Classical Logic Duminda Wijesekera
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What Other Forms of Logics are there?
Can enrich 1st Order Logic by introducing more connectives Example: Modal Logic: Adding two new constructors to reason about possibility and nasality ◊ and Syntax: If f is a formula so are ◊ f and f ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Applications of Modal Logic
Dynamic Logic Temporal Logics Theories of Motion Theories of Change Theories of Knowledge Applications in security Access control Delegation See separate transparency on Modal, Dynamic and Temporal Logics ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Other Forms to Enhance 1st Order Logic
Add Richer Quantifier Structures: Quantify over subsets of structures and/or relation symbols Add quantifiers such as there exists infinitely many, or for infinitely many. Example: x y factors(x,y) y prime(y) ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Apply existing connectives many more times – infinitary logics
If f1, … fn, .. are formulas so are ^ {i=1} fi v {i=1} fi Different from having infinitary quantifiers ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Restrictions based on Semantics
Can specify the size of models Infinitary but large models Such as only those beyond a given cardinal א1 Finite model theory Many applications in computer science Quantifiers take an entirely different meaning ISA 780/SWE 623Classical Logic Duminda Wijesekera
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Those that reject basic assumptions of Classical Logic
Intuitionism Why is (A → B) v (B → A) a tautology ? Pigenhole principle in classical truth values Similar argument for (A1 → A2) v (A2 → A3) …. (An → An+1) v (An+1 → A1) Consequence: P v P does not have to be true! Infinitely many truth values Leads to Kripke Semantics with worlds with increasing truth values Used in the semantics of programming languages and type theories ISA 780/SWE 623Classical Logic Duminda Wijesekera
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