Flashcards in Textbook Summary Boxes Deck (14)

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## Section 1.1: Individual Constants (3)

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- Every individual constant must name an (actually existing) object.

- No individual constant can name more than one object.

- An object can have more than one name, or no name at all.

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## Section 1.2: Predicate Symbols (2)

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- Every predicate symbol comes with a single, fixed "arity," a number that tells you how many names it needs to form an atomic sentence.

- Every predicate is interpreted by a determinate property or relation of the same arity as the predicate.

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## Section 1.3: Atomic Sentences (3)

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- Atomic sentences are formed by putting a predicate of arity n in front of n names (enclosed in parentheses and separated by commas).

- Atomic sentences are built from the identity predicate, =, using infix notation: the arguments are placed on either side of the predicate.

- The order of the names is crucial in forming atomic sentences.

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## Section 2.1: Valid & Sound Arguments (4)

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- An argument is a series of statements in which one, called the conclusion, is meant to be a consequence of the others, called the premises.

- An argument is valid if the conclusion must be true in any circumstances in which the premises are true.

- We say that the conclusion of a logically valid argument is a logical consequence of its premises.

- An argument is sound if it is valid and the premises are all true.

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## Section 2.2: Methods of Proof (2)

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- A proof of a statement S from premises P1, ... , Pn is a step-by-step demonstration which shows that S must be true in any circumstances in which the premises P1, ... , Pn are all true.

- Informal and formal proofs differ in style, not in rigor.

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## Section 2.2: Methods of Proof - Principles of the Identity Relation (4)

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- = Elim: If b = c, then whatever holds of b holds of c. This is also known as the indiscernibility of identicals.

- = Intro: Sentences of the form b = b are always true (in FOL). This is also known as the reflexivity of identity.

- Symmetry of Identity: If b = c, then c = b.

- Transitivity of Identity: If a = b and b = c, then a = c

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## Section 2.5: Demonstrating Nonconsequence (2)

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- To demonstrate the invalidity of an argument with premises P1, ... , Pn and conclusion Q, find a counterexample: a possible circumstance that makes P1, ... , Pn all true but Q false.

- Such a counterexample shows that Q is not a consequence of P1, ... , Pn.

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## Section 3.1: Negation Symbol (3)

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- If P is a sentence of FOL, then so is ¬P.

- The sentence ¬P is true if and only if P is not true.

- A sentence that is ether atomic or the negation of an atomic sentence is called a literal.

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## Section 3.2: Conjunction Symbol (2)

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- If P and Q are sentences of FOL, then so is P ∧ Q.

- The sentence P ∧ Q is true if and only if both P and Q are true.

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## Section 3.3: Disjunction Symbol (2)

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- If P and Q are sentences of FOL, then so is P ∨ Q.

- The sentence P ∨ Q is true if and only if P is true or Q is true (or both are true).

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## Section 3.6: Equivalent Ways of Saying Things - DeMorgan's Laws (3)

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- Double negation: ¬¬P ⇔ P

- DeMorgan: ¬(P ∧ Q) ⇔ (¬P ∨ ¬Q)

- DeMorgan: ¬(P ∨ Q) ⇔ (¬P ∧ ¬Q)

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## Section 4.1: Tautologies & Logical Truths - Let S be a sentence of FOL built up from atomic sentences by means of truth-functional connectives alone. A truth table for S shows how the truth of S depends on the truth of its atomic parts. (4)

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- S is a tautology if and only if every row of the truth table assigns TRUE to S.

- If S is a tautology, then S is a logical truth (that is, logically necessary).

- Some logical truths are not tautologies.

- S is TT-possible if and only if at least one row of the truth table assigns TRUE to S.

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## Section 4.2: Logical & Tautological Equivalence - Let S and S' be sentences of FOL built up from atomic sentences by means of truth-functional connectives alone. To test for tautological equivalence, we construct a joint truth table for the two sentences. (3)

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- S and S' are tautologically equivalent if and only if every row of the joint truth table assigns the same values to S and S'.

- If S and S' are tautologically equivalent, then they are logically equivalent.

- Some logically equivalent sentences are not tautologically equivalent.

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