Material conditional

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The material conditional, also known as the material implication or truth functional conditional, expresses a property of certain conditionals in logic. In propositional logic, it expresses a binary truth function ⊂ from truth-values to truth-values. In predicate logic, it can be viewed as a subset relation between the extension of (possibly complex) predicates. In symbols, a material conditional is written as one of the following:

$X supset Y$ ,
$X to Y$ , and sometimes
$X Rightarrow Y$

The material conditional is false when X is true and Y is false - otherwise, it is true. (Here, X and Y are variables ranging over formulæ of a formal theory.) We call X the antecedent, and Y the consequent. The material conditional is also commonly referred to as material implication with the understanding that the antecedent (X) materially implies the consequent (Y).

Part of the meaning of the material conditional is encapsulated by the English "if condition then consequence" construction, where the condition and consequence are to be filled with English sentences. However, this construction may not imply the other part of material conditional, which is that a false condition implies nothing about the truth or falsity of the consequence.

An exact encapsulation of the material conditional X → Y is "it's false that X be true while Y false" — i.e. in symbols, $neg(X and neg Y)$ . Arguably this is more intuitive than its logically equivalent disjunction ¬X ∨ Y.

Definition

Logical implication and the material conditional are both associated with an operation on two logical values, typically the values of two propositions, that produces a value of false just in case the first operand is true and the second operand is false.

Truth table

The truth table associated with the material conditional if p then q (symbolized as p → q) and the logical implication p implies q (symbolized as p ⇒ q) is as follows:


p	q	→
T	T	T
T	F	F
F	T	T
F	F	T

Johnston diagram

The Johnston diagram of

A Rightarrow B

- "If A then B" - where the white portion indicates the space in which the relation is false.

Formal properties

The material conditional is not to be confused with the entailment relation ⊨ (which is used here as a name for itself). But there is a close relationship between the two in most logics, including classical logic which we only consider here. For example, the following principles hold:

If $Gammamodelspsi$ then $emptysetmodelsphi_1landdotslandphi_nsubsetpsi$ for some $phi_1,dots,phi_ninGamma$ . (This is a particular form of the deduction theorem.)

The converse of the above

Both ⊂ and ⊨ are monotonic; i.e., if $Gammamodelspsi$ then $DeltacupGammamodelspsi$ , and if $phisubsetpsi$ then $(philandalpha)subsetpsi$ for any α, Δ. (In terms of structural rules, this is often referred to as weakening or thinning.)

These principles do not hold in all logics, however. Obviously they do not hold in non-monotonic logics, nor do they hold in relevance logics.

Other properties of implication:

distributivity: $s rightarrow (p rightarrow q) rightarrow ((s rightarrow p) rightarrow (s rightarrow q))$

transitivity: ( $a rightarrow b) rightarrow ((b rightarrow c) rightarrow (a rightarrow c))$

commutativity: ( $a rightarrow (b rightarrow c)) equiv (b rightarrow (a rightarrow c))$

idempotency: $a rightarrow a$

truth preserving : The interpretation under which all variables are assigned a truth value of 'true' produces a truth value of 'true' as a result of material implication.

Philosophical problems with material conditional

The truth function ⊂ does not correspond exactly to the English 'if...then...' construction. For example, any material conditional statement with a false antecedent is true. So the statement "if 2 is odd then 2 is even" is true. Similarly, any material conditional with a true consequent is true. So the statement, "if Pigs fly then Paris is in France" is true. These problems are known as the paradoxes of material implication, though they are not really paradoxes in the strict sense; that is, they do not elicit logical contradictions.

There are various kinds of conditionals in English; e.g., there is the indicative conditional and the subjunctive or counterfactual conditional. The latter do not have the same truth conditions as the material conditional. For an overview of some the various analyses, formal and informal, of conditionals, see the "References" section below.

References

Brown, Frank Markham (2003), Boolean Reasoning: The Logic of Boolean Equations, 1st edition, Kluwer Academic Publishers, Norwell, MA. 2nd edition, Dover Publications, Mineola, NY, 2003.

Edgington, Dorothy (2001), "Conditionals", in Lou Goble (ed.), The Blackwell Guide to Philosophical Logic, Blackwell.

Edgington, Dorothy (2006), "Conditionals", in Edward N. Zalta (ed.), The Stanford Encyclopedia of Philosophy, Eprint

Quine, W.V. (1982), Methods of Logic, (1st ed. 1950), (2nd ed. 1959), (3rd ed. 1972), 4th edition, Harvard University Press, Cambridge, MA.