↑ arvau

Wikipedia / Material conditional

The material conditional (also known as material implication) is a binary operation commonly used in logic is interpreted is false.

Material implication is used in all the basic systems of classical logic as well as some nonclassical logics. It is assumed as a model of correct conditional reasoning within mathematics and serves as the basis for commands in many programming languages. However, many logics replace material implication with other operators such as the strict conditional and the variably strict conditional. Due to the paradoxes of material implication and related problems, material implication is not generally considered a viable analysis of conditional sentences in natural language.

Notation

(). ( and respectively). In the prefixed Polish notation is referred to as the antecedent is termed the consequent.

History

In Arithmetices Principia: Nova Methodo Exposita (1889), Peano with the symbol Ɔ, which is the opposite of C.. Hilbert expressed the proposition "If A, then B in 1918. Russell followed Peano in his Principia Mathematica (1910–1913), in which he expressed the proposition "If A, then B. Following Russell, Gentzen expressed the proposition "If A, then B. Heyting expressed the proposition "If A, then B with a right-pointing arrow. Bourbaki expressed the proposition "If A, then B in 1954.

Semantics

Truth table

From a classical semantic perspective, material implication is the binary truth functional operator which returns "true" unless its first argument is true and its second argument is false. This semantics can be shown graphically in the following truth table: .

is false, are called "vacuous truths". Examples are ...

  • false: "If Marie Curie is a sister of Galileo Galilei, then Galileo Galilei is a brother of Marie Curie."

  • true: "If Marie Curie is a sister of Galileo Galilei, then Marie Curie has a sibling."

Analytic tableaux

are called f-implicational. In classical logic (negation (conjunction (disjunction (equivalence (falsity):

The validity of f-implicational formulas can be semantically established by the method of analytic tableaux. The logical rules are

  1. {| style="border: none; border-spacing: 1px; border-collapse: separate;"

: Close the branch (contradiction) : Do nothing (since it just asserts no contradiction) |}

        F[p → ((p → ⊥) → ⊥)]
         |
        T[p]
        F[(p → ⊥) → ⊥]
         |
        T[p → ⊥]
        F[⊥]
┌────────┴────────┐

F[p] T[⊥]

|                 |

CONTRADICTION CONTRADICTION (T[p], F[p]) (⊥ is true) 

        F[((p → ⊥) → ⊥) → p]
         |
        T[(p → ⊥) → ⊥]
        F[p]
┌────────┴────────┐

F[p → ⊥] T[⊥]

|                 |

T[p] CONTRADICTION (⊥ is true) F[⊥]

|

CONTRADICTION (T[p], F[p]) Hilbert-style proofs can be found here or here. 

1. F[(p → q) → ((q → r) → (p → r))]
             |                       // from 1
         2. T[p → q]
         3. F[(q → r) → (p → r)]
             |                       // from 3
         4. T[q → r]
         5. F[p → r]
             |                       // from 5
         6. T[p]
         7. F[r]
    ┌────────┴────────┐              // from 2

8a. F[p] 8b. T[q]

    X        ┌────────┴────────┐     // from 4
        9a. F[q]          9b. T[r]
             X                 X

A Hilbert-style proof can be found here.

Syntactical properties

The semantic definition by truth tables does not permit the examination of structurally identical propositional forms in various logical systemss, where different properties may be demonstrated. The language considered here is restricted to f-implicational formulas.

Consider the following (candidate) natural deduction rules.

  • Minimal logic: By limiting the natural deduction rules to Implication IntroductionI) and Implication EliminationE), one obtains (the implicational fragment of) minimal logic (as defined by Johansson).

A selection of theorems (classical logic)

In classical logic material implication validates the following:

Similarly, on classical interpretations of the other connectives, material implication validates the following entailmentss:

Tautologies involving material implication include:

The relationship between the material conditional and logical consequence

The material conditional is a sentential connective within a formal language. It should not be confused with the relation of logical consequence (also called logical implication or entailment), which is standardly treated as a relation between sentences expressed in a metalanguage.

The relationship between the material conditional and the logical consequence relation is given by the deduction theorem.

together with A logically implies B.

is empty, this reduces to:

This states A logically implies B is a theorem of the logic.

Many textbooks reserve the term logical consequence (or logical implication) for the semantic consequence. In which case the relation becomes

A logically implies B is a tautology.

Discrepancies with natural language

Material implication does not closely match the usage of conditional sentences in natural language. For example, even though material conditionals with false antecedents are vacuously true, the natural language statement "If 8 is odd, then 3 is prime" is typically judged false. Similarly, any material conditional with a true consequent is itself true, but speakers typically reject sentences such as "If I have a penny in my pocket, then Paris is in France". These classic problems have been called the paradoxes of material implication. In addition to the paradoxes, a variety of other arguments have been given against a material implication analysis. For instance, counterfactual conditionals would all be vacuously true on such an account, when in fact some are false.

In the mid-20th century, a number of researchers including H. P. Grice and Frank Jackson proposed that pragmatic principles could explain the discrepancies between natural language conditionals and the material conditional. On their accounts, conditionals denote material implication but end up conveying additional information when they interact with conversational norms such as Grice's maxims. Recent work in formal semantics and philosophy of language has generally eschewed material implication as an analysis for natural-language conditionals. In particular, such work has often rejected the assumption that natural-language conditionals are truth functional in the sense that the truth value of "If P, then Q" is determined solely by the truth values of P and Q. Thus semantic analyses of conditionals typically propose alternative interpretations built on foundations such as modal logic, relevance logic, probability theory, and causal models.

Similar discrepancies have been observed by psychologists studying conditional reasoning, for instance, by the notorious Wason selection task study, where less than 10% of participants reasoned according to the material conditional. Some researchers have interpreted this result as a failure of the participants to conform to normative laws of reasoning, while others interpret the participants as reasoning normatively according to nonclassical laws.

See also

Conditionals

Notes

Bibliography

Further reading