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varieties of necessity tautological equivalence tautological consequence LECTURE 10

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Let’s call a sentence Tarski’s World-necessary (TW-necessary) just in case it is true in all the worlds that can possibly be constructed in Tarski’s World. What’s the relationship between logical truth and TW-necessity? All logical truths are TW-necessary. Some TW-necessities are not logical truths, e.g., ‘Tet(a) ∨ Cube(a) ∨ Dodec(a).’ So the logical necessities are a strict subset of the TW- necessities. LOGICAL TRUTH AND TARSKI’S WORLD NECESSITIES

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For each of the following sentences, say where it goes in the following Euler circle diagram EXERCISE Larger(a,b) ∨ ¬Larger(a,b) 6. Larger(a,b) ∨ Smaller (a,b)

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‘¬[¬Tet(a) ∧ ¬Cube(a) ∧ ¬Dodec(a)]’ is A.A tautology B.A TW-necessity C.A and B D.None of the above I>CLICKER QUESTION

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LOGICAL AND TAUTOLOGICAL EQUIVALENCE

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Our informal definition of equivalence, like our informal definition of logical truth, also referred to possible worlds: P and Q are equivalent if and only if they are true in all and only the same possible worlds. With our different conceptions of possibility and necessity, we can distinguish different kinds of equivalence: tautological equivalence logical equivalence, and TW-equivalence. EQUIVALENCE

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To give a more precise definition of tautological equivalence, we will use the notion of sentences’ joint truth tables. The joint truth table for A and B is the truth table with both sentences to the right of the reference columns. Sentences A and B are tautologically equivalent if and only if at each row, the truth value under A’s main connective is the same as the truth value under B’s main connective. TAUTOLOGICAL EQUIVALENCE AA ∨ ¬AA ∧ ¬A T T F F F F T T F T

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DeMorgan’s Laws Double Negation SOME IMPORTANT LOGICAL EQUIVALENCES

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All tautological equivalences are logical equivalences. But remember how not all logical truths were tautologies? Similarly, not all tautological equivalences are logical equivalences. Example: ‘a = b’ is logically but not tautologically equivalent to ‘¬FrontOf(a,b).’ TAUTOLOGICAL EQUIVALENCE AND LOGICAL EQUIVALENCE

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