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ece 720 intelligent web: ontology and beyond
lecture 12: propositional logic – part I
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propositions and connectives …
two-valued logic – every sentence is either true or false some sentences are minimal – no proper part which is also a sentence others – can be taken apart into smaller parts we can build larger sentences from smaller ones by using connectives ece 720, winter ‘12
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propositions and connectives …
connectives – each has one or more meanings in natural language – need for precise, formal language If I open the window then 1+3=4. John is working or he is at home. Euclid was a Greek or a mathematician. ece 720, winter ‘12
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propositional logic … can do …
logic that deals only with propositions (sentences, statements) if it is raining then people carry their umbrellas raised people are not carrying their umbrellas raised it is not raining ece 720, winter ‘12
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propositional logic … cannot do …
if it is raining then everyone will have their umbrellas up John is a person John does not have his umbrella up (it is not raining) no way to substitute the name of the individual John into the general rule about everyone ece 720, winter ‘12
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propositional logic … we will look at three different versions
Hilbert system (the view that logic is about proofs) Gentzen system and tableau (or Beth) system (the view that is about consistency and entailment) ece 720, winter ‘12
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propositional logic … two stages of formalization
present a formal language specify a procedure for obtaining valid/true propositions ece 720, winter ‘12
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propositional logic Hilbert system
PROP_H the syntax is extremely simple a set of names for prepositions single logical operator a single rule for combining simple expressions via logical operator (using brackets to avoid ambiguity) ece 720, winter ‘12
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propositional logic Hilbert system
PROP_H is a system which is tailored for talking about what can and what cannot be proved within the language, rather than for actually saying things and exploring entailments ece 720, winter ‘12
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propositional logic Hilbert system
language … propositions names: p, q, r, …, p0, p1, p2, … a name for “false”: ece 720, winter ‘12
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propositional logic Hilbert system
language … the connective: (intended to be read “implies”) single combining rule: if A and B are expressions then so is A B (A, B stand for arbitrary expressions of the language) ece 720, winter ‘12
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propositional logic Hilbert system - semantics
is given by explaining the meaning of the basic formulae - the proposition letters and is given in terms of two objects T and F is given by providing a function V called valuation (it defines meaning of expressions) ece 720, winter ‘12
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propositional logic Hilbert system - semantics
V(P) – maps P (basic proposition letters and ) to {T, F} expr - denotes the meaning assigned to expr so, how to find out the value of complex expressions (built from simple ones using , i.e., A B ) …? ece 720, winter ‘12
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propositional logic Hilbert system - semantics
answer – truth tables A B A B T T T T F F F T T F F T ece 720, winter ‘12
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propositional logic Hilbert system - semantics
a truth table of complex expressions A B B A A (B A)) T T T T T F T T F T F T F F T T ece 720, winter ‘12
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propositional logic Hilbert system - semantics
negation - A A T F F F F T ece 720, winter ‘12
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propositional logic Hilbert system - semantics
disjunction A B A A B T T F T T F F T F T T T F F T F ece 720, winter ‘12
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propositional logic Hilbert system - semantics
conjunction A B A B A B ( A B) T T F F F T T F F T T F F T T F T F F F T T T F ece 720, winter ‘12
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propositional logic Hilbert system – inference rules
only one inference rule for any expressions A and B: from A and A B we can infer B it is known as MODUS PONENS (MP) ece 720, winter ‘12
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propositional logic Hilbert system – inference rules
+ three axioms (expressions that are accepted as being true under any evaluation) PH1: A (B A) PH2: [A (B C)] [(A B) (A C)] PH3: ( A) A ece 720, winter ‘12
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propositional logic Hilbert system – inference rules
a proof of a conclusion C from hypotheses H1, H2, …, Hk is a sequence of expressions E1, E2, …, En obeying the following conditions: C is En Each Ei is either an axiom, or one of the Hj or is a result of applying the inference rule MP to Ek and El where k and l are less then i ece 720, winter ‘12
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propositional logic Hilbert system – inference rules
proof of r from p hypotheses: p q, q r 1 p (hypothesis) 2 p q (hypothesis) 3 q (from 1 and 2 by MP) 4 q r (hypothesis) 5 r (from 3 and 4 by MP) ece 720, winter ‘12
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propositional logic Hilbert system – inference rules
p, p q, q r r is a meta-symbol indicating that proof exists if we have a proof of a conclusion from an empty set of hypotheses – a formula which can be proved in such a way is called a theorem ece 720, winter ‘12
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propositional logic Hilbert system – inference rules
proof of A A 1 [A ((A A) A)] [(A (A A)) (A A)] (PH2) 2 [A ((A A) A)] (PH1) 3 [A (A A)] (A A) (MP on 1, 2) 4 (A (A A)) (PH1) 5 (A A) (MP on 3, 4) ece 720, winter ‘12
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propositional logic Hilbert system – inference rules
deduction theorem if A1, …, An B then A1, …, An-1 An B ece 720, winter ‘12
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propositional logic Hilbert system – inference rules
proof of A A 1 A, A (MP) 2 A (A ) (DT) 3 A ((A ) (DT) 4 A A (A A) ece 720, winter ‘12
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propositional logic Hilbert system – inference rules
proof of A 1 , A 2 (A ) (DT) 3 [(A ) ] A (PH3) 4 A (MP on 2, 3) 5 A (DT) ece 720, winter ‘12
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propositional logic Hilbert system – soundness
would like to be reassured that Prop_H will not permit us to infer conclusions which can be false when their supporting hypotheses are true (otherwise, we would be able to prove things which are not true) ece 720, winter ‘12
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propositional logic Hilbert system – soundness definition
if A1, …, An A then A = T for any valuation function V for which all the Ai are T ece 720, winter ‘12
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propositional logic Hilbert system – completeness
would like to know that if some conclusion is always true whenever every member of a given set of hypotheses is true, then there is a proof of the conclusion from the hypotheses ece 720, winter ‘12
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propositional logic Hilbert system – completeness definition
if A is valid then A ece 720, winter ‘12
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propositional logic Hilbert system – consistency definition
there is no proof of ece 720, winter ‘12
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propositional logic Hilbert system – decidability definition
there is a mechanical procedure which can decide for any A whether or not A ece 720, winter ‘12
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