The material conditional (also known as material implication, material consequence, or simply implication, implies, or conditional) is a logical connective (or a binary operator) that is often symbolized by a forward arrow "->". The material conditional is used to form statements of the form (termed a conditional statement) which is read as "if p then q". Unlike the English construction "if ... then ...", the material conditional statement does not conventionally specify a causal relationship between p and q; "p is the cause and q is the consequence from it" is not a generally valid interpretation of . It merely means "if p is true then q is also true" such that the statement is false only when p is true and q is false. In a bivalent truth table of , if p is false then is true regardless of whether q is true or false (Latin phrase: ex falso quod libet) since (1) p -> q is always true as long q is true and (2) is true when both p and q are false. This truth table is useful to prove some mathematical theorems (e.g., defining a subset).
The material conditional is also symbolized using:
With respect to the material conditionals above:
Conditional statements may be nested such that either or both of the antecedent or the consequent may themselves be conditional statements. In the example , meaning "if the truth of p implies the truth of q then the truth of r implies the truth of s", both the antecedent and the consequent are conditional statements.
In classical logic, is logically equivalent to and by De Morgan's Law logically equivalent to . Whereas, in minimal logic (and therefore also intuitionistic logic) only logically entails ; and in intuitionistic logic (but not minimal logic) entails .
The term "material implication" was coined by Russell, who made a distinction between formal and material implication.
Here's a quote from the Principia:
[W]herever [...] one particular proposition is deduced from another, material implication is involved, though as a rule the material implication may be regarded as a particular instance of some formal implication, obtained by giving some constant value to the variable or variables involved in the said formal implication.
So material implication concerns implication between particular propositions, whereas formal implication is supposed to be more general. It doesn't have much to do with matter as in physical stuff, it is material only in the sense of being a particular instance of something.
There are other conditionals which are not material, like the subjunctive conditional (e.g. "If Oswald had not shot Kennedy, then someone else would have").
Logicians have many different views on the nature of material implication and approaches to explain its sense.
The compound is false if and only if p is true and q is false. By the same stroke, is true if and only if either p is false or q is true (or both). The -> symbol is a function that uses pairs of truth values of the components p, q (e.g., p is True, q is True ... p is False, q is False) and maps it to the truth values of the compound . The truth value of is a function of the truth values of its components (p, q). Hence, this interpretation is called truth-functional.
The compound is logically equivalent also to (either not p, or q (or both)), and to (if not q then not p). It is, however, not equivalent to , which is instead equivalent to .
The truth table associated with the material conditional p->q is identical to that of ¬p?q. It is as follows:
In Boolean algebra, true and false can be denoted as 1 and 0 respectively with an equivalent table.
The material conditional can be considered as a symbol of a formal theory, taken as a set of sentences, satisfying all the classical inferences involving ->, in particular the following characteristic rules:
Unlike the truth-functional one, this approach to logical connectives permits the examination of structurally identical propositional forms in various logical systems, where somewhat different properties may be demonstrated. For example, in intuitionistic logic which rejects proofs by contraposition as valid rules of inference, (p -> q) => ¬p ? q is not a propositional theorem, but the material conditional is used to define negation.
"It is not the case that given true, won't be true."
"Impossible without ." The word "without" doesn't mean "because of the lack of", but "in absence of the consequent".
When studying logic formally, the material conditional is distinguished from the semantic consequence relation . We say if every interpretation that makes A true also makes B true. However, there is a close relationship between the two in most logics, including classical logic. For example, the following principles hold:
Other properties of implication (the following expressions are always true, for any logical values of variables):
Comparison of Boolean truth tables shows that is equivalent to , and one is an equivalent replacement for the other in classical logic. See material implication (rule of inference).
Outside of mathematics, it is a matter of some controversy as to whether the truth function for material implication provides an adequate treatment of conditional statements in a natural language such as English, i.e., indicative conditionals and counterfactual conditionals. An indicative conditional is a sentence in the indicative mood with a conditional clause attached. A counterfactual conditional is a false-to-fact sentence in the subjunctive mood. That is to say, critics argue that in some non-mathematical cases, the truth value of a compound statement, "if p then q", is not adequately determined by the truth values of p and q. Examples of non-truth-functional statements include: "q because p", "p before q" and "it is possible that p".
"[Of] the sixteen possible truth-functions of A and B, material implication is the only serious candidate. First, it is uncontroversial that when A is true and B is false, "If A, B" is false. A basic rule of inference is modus ponens: from "If A, B" and A, we can infer B. If it were possible to have A true, B false and "If A, B" true, this inference would be invalid. Second, it is uncontroversial that "If A, B" is sometimes true when A and B are respectively (true, true), or (false, true), or (false, false)... Non-truth-functional accounts agree that "If A, B" is false when A is true and B is false; and they agree that the conditional is sometimes true for the other three combinations of truth-values for the components; but they deny that the conditional is always true in each of these three cases. Some agree with the truth-functionalist that when A and B are both true, "If A, B" must be true. Some do not, demanding a further relation between the facts that A and that B."
The truth-functional theory of the conditional was integral to Frege's new logic (1879). It was taken up enthusiastically by Russell (who called it "material implication"), Wittgenstein in the Tractatus, and the logical positivists, and it is now found in every logic text. It is the first theory of conditionals which students encounter. Typically, it does not strike students as obviously correct. It is logic's first surprise. Yet, as the textbooks testify, it does a creditable job in many circumstances. And it has many defenders. It is a strikingly simple theory: "If A, B" is false when A is true and B is false. In all other cases, "If A, B" is true. It is thus equivalent to "~(A&~B)" and to "~A or B". "A ? B" has, by stipulation, these truth conditions.
The meaning of the material conditional can sometimes be used in the English "if condition then consequence" construction (a kind of conditional sentence), where condition and consequence are to be filled with English sentences. However, this construction also implies a "reasonable" connection between the condition (protasis) and consequence (apodosis) (see Connexive logic).
The material conditional can yield some unexpected truths when expressed in natural language. For example, any material conditional statement with a false antecedent is true (see vacuous truth). 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 I have a penny in my pocket then Paris is in France" is always true, regardless of whether or not there is a penny in my pocket. 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. These unexpected truths arise because speakers of English (and other natural languages) are tempted to equivocate between the material conditional and the indicative conditional, or other conditional statements, like the counterfactual conditional and the material biconditional.
It is not surprising that a rigorously defined truth-functional operator does not correspond exactly to all notions of implication or otherwise expressed by "if ... then ..." sentences in natural languages. For an overview of some of the various analyses, formal and informal, of conditionals, see the "References" section below. Relevance logic attempts to capture these alternate concepts of implication that material implication glosses over.