Propositional calculus

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Template:Transformation rules Propositional calculus (also called propositional logic, sentential calculus, or sentential logic) is the branch of mathematical logic concerned with the study of propositions (whether they are true or false) and formed by other propositions with the use of logical connectives, and how their value depends on the truth value of their components. Logical connectives are found in natural languages. In English for example, some examples are "and" (conjunction), "or" (disjunction), "not” (negation) and "if" (but only when used to denote material conditional).

The following is an example of a very simple inference within the scope of propositional logic:

Premise 1: If it's raining then it's cloudy.
Premise 2: It's raining.
Conclusion: It's cloudy.

Both premises and the conclusions are propositions. The premises are taken for granted and then with the application of modus ponens (an inference rule) the conclusion follows.

As propositional logic is not concerned with the structure of propositions beyond the point where they can't be decomposed anymore by logical connectives, this inference can be restated replacing those atomic statements with statement letters, which are interpreted as variables representing statements:

Premise 1:
Premise 2:
Conclusion:

The same can be stated succinctly in the following way:

When Template:Mvar is interpreted as “It's raining” and Template:Mvar as “it's cloudy” the above symbolic expressions can be seen to exactly correspond with the original expression in natural language. Not only that, but they will also correspond with any other inference of this form, which will be valid on the same basis that this inference is.

Propositional logic may be studied through a formal system in which formulas of a formal language may be interpreted to represent propositions. A system of inference rules and axioms allows certain formulas to be derived. These derived formulas are called theorems and may be interpreted to be true propositions. A constructed sequence of such formulas is known as a derivation or proof and the last formula of the sequence is the theorem. The derivation may be interpreted as proof of the proposition represented by the theorem.

When a formal system is used to represent formal logic, only statement letters are represented directly. The natural language propositions that arise when they're interpreted are outside the scope of the system, and the relation between the formal system and its interpretation is likewise outside the formal system itself.

Usually in truth-functional propositional logic, formulas are interpreted as having either a truth value of true or a truth value of false.Template:Clarify Truth-functional propositional logic and systems isomorphic to it, are considered to be zeroth-order logic.

History

{{#invoke:main|main}} Although propositional logic (which is interchangeable with propositional calculus) had been hinted by earlier philosophers, it was developed into a formal logic by Chrysippus in the 3rd century BC[1] and expanded by the Stoics. The logic was focused on propositions. This advancement was different from the traditional syllogistic logic which was focused on terms. However, later in antiquity, the propositional logic developed by the Stoics was no longer understood Template:Who. Consequently, the system was essentially reinvented by Peter Abelard in the 12th century.[2]

Propositional logic was eventually refined using symbolic logic. The 17th/18th century philosopher Gottfried Leibniz has been credited with being the founder of symbolic logic for his work with the calculus ratiocinator. Although his work was the first of its kind, it was unknown to the larger logical community. Consequently, many of the advances achieved by Leibniz were reachieved by logicians like George Boole and Augustus De Morgan completely independent of Leibniz.[3]

Just as propositional logic can be considered an advancement from the earlier syllogistic logic, Gottlob Frege's predicate logic was an advancement from the earlier propositional logic. One author describes predicate logic as combining "the distinctive features of syllogistic logic and propositional logic."[4] Consequently, predicate logic ushered in a new era in logic's history; however, advances in propositional logic were still made after Frege, including Natural Deduction, Truth-Trees and Truth-Tables. Natural deduction was invented by Gerhard Gentzen and Jan Łukasiewicz. Truth-Trees were invented by Evert Willem Beth.[5] The invention of truth-tables, however, is of controversial attribution.

Within works by Frege[6] and Bertrand Russell,[7] one finds ideas influential in bringing about the notion of truth tables. The actual 'tabular structure' (being formatted as a table), itself, is generally credited to either Ludwig Wittgenstein or Emil Post (or both, independently).[6] Besides Frege and Russell, others credited with having ideas preceding truth-tables include Philo, Boole, Charles Sanders Peirce, and Ernst Schröder. Others credited with the tabular structure include Łukasiewicz, Schröder, Alfred North Whitehead, William Stanley Jevons, John Venn, and Clarence Irving Lewis.[7] Ultimately, some have concluded, like John Shosky, that "It is far from clear that any one person should be given the title of 'inventor' of truth-tables.".[7]

Terminology

In general terms, a calculus is a formal system that consists of a set of syntactic expressions (well-formed formulas), a distinguished subset of these expressions (axioms), plus a set of formal rules that define a specific binary relation, intended to be interpreted to be logical equivalence, on the space of expressions.

When the formal system is intended to be a logical system, the expressions are meant to be interpreted to be statements, and the rules, known to be inference rules, are typically intended to be truth-preserving. In this setting, the rules (which may include axioms) can then be used to derive ("infer") formulas representing true statements from given formulas representing true statements.

The set of axioms may be empty, a nonempty finite set, a countably infinite set, or be given by axiom schemata. A formal grammar recursively defines the expressions and well-formed formulas of the language. In addition a semantics may be given which defines truth and valuations (or interpretations).

The language of a propositional calculus consists of

  1. a set of primitive symbols, variously referred to be atomic formulas, placeholders, proposition letters, or variables, and
  2. a set of operator symbols, variously interpreted to be logical operators or logical connectives.

A well-formed formula is any atomic formula, or any formula that can be built up from atomic formulas by means of operator symbols according to the rules of the grammar.

Mathematicians sometimes distinguish between propositional constants, propositional variables, and schemata. Propositional constants represent some particular proposition, while propositional variables range over the set of all atomic propositions. Schemata, however, range over all propositions. It is common to represent propositional constants by Template:Mvar, Template:Mvar, and Template:Mvar, propositional variables by Template:Mvar, Template:Mvar, and Template:Mvar, and schematic letters are often Greek letters, most often Template:Mvar, Template:Mvar, and Template:Mvar.

Basic concepts

The following outlines a standard propositional calculus. Many different formulations exist which are all more or less equivalent but differ in the details of:

  1. their language, that is, the particular collection of primitive symbols and operator symbols,
  2. the set of axioms, or distinguished formulas, and
  3. the set of inference rules.

Any given proposition may be represented with a letter called a 'propositional constant', analogous to representing a number by a letter in mathematics, for instance, a = 5. All propositions require exactly one of two truth-values: true or false. For example, let Template:Mvar be the proposition that it is raining outside. This will be true (Template:Mvar) if it is raining outside and false otherwise (¬P).

The conjunction of Template:Mvar and Template:Mvar is true in case 1 and is false otherwise. Where Template:Mvar is the proposition that it is raining outside and Template:Mvar is the proposition that a cold-front is over Kansas, PQ is true when it is raining outside and there is a cold-front over Kansas. If it is not raining outside, then Template:Mvar is false; and if there is no cold-front over Kansas, then PQ is false.
  • Disjunction resembles conjunction in that it forms a proposition out of two simpler propositions. We write it PQ, and it is read "Template:Mvar or Template:Mvar". It expresses that either Template:Mvar or Template:Mvar is true. Thus, in the cases listed above, the disjunction of Template:Mvar and Template:Mvar is true in all cases except 4. Using the example above, the disjunction expresses that it is either raining outside or there is a cold front over Kansas. (Note, this use of disjunction is supposed to resemble the use of the English word "or". However, it is most like the English inclusive "or", which can be used to express the truth of at least one of two propositions. It is not like the English exclusive "or", which expresses the truth of exactly one of two propositions. That is to say, the exclusive "or" is false when both Template:Mvar and Template:Mvar are true (case 1). An example of the exclusive or is: You may have a bagel or a pastry, but not both. Often in natural language, given the appropriate context, the addendum "but not both" is omitted but implied. In mathematics, however, "or" is always inclusive or; if exclusive or is meant it will be specified, possibly by "xor".)
  • Material conditional also joins two simpler propositions, and we write PQ, which is read "if Template:Mvar then Template:Mvar". The proposition to the left of the arrow is called the antecedent and the proposition to the right is called the consequent. (There is no such designation for conjunction or disjunction, since they are commutative operations.) It expresses that Template:Mvar is true whenever Template:Mvar is true. Thus it is true in every case above except case 2, because this is the only case when Template:Mvar is true but Template:Mvar is not. Using the example, if Template:Mvar then Template:Mvar expresses that if it is raining outside then there is a cold-front over Kansas. The material conditional is often confused with physical causation. The material conditional, however, only relates two propositions by their truth-values—which is not the relation of cause and effect. It is contentious in the literature whether the material implication represents logical causation.
  • Biconditional joins two simpler propositions, and we write PQ, which is read "Template:Mvar if and only if Template:Mvar". It expresses that Template:Mvar and Template:Mvar have the same truth-value, thus Template:Mvar if and only if Template:Mvar is true in cases 1 and 4, and false otherwise.

It is extremely helpful to look at the truth tables for these different operators, as well as the method of analytic tableaux.

Closure under operations

Propositional logic is closed under truth-functional connectives. That is to say, for any proposition Template:Mvar, ¬φ is also a proposition. Likewise, for any propositions Template:Mvar and Template:Mvar, φψ is a proposition, and similarly for disjunction, conditional, and biconditional. This implies that, for instance, φψ is a proposition, and so it can be conjoined with another proposition. In order to represent this, we need to use parentheses to indicate which proposition is conjoined with which. For instance, PQR is not a well-formed formula, because we do not know if we are conjoining PQ with Template:Mvar or if we are conjoining Template:Mvar with QR. Thus we must write either (PQ) ∧ R to represent the former, or P ∧ (QR) to represent the latter. By evaluating the truth conditions, we see that both expressions have the same truth conditions (will be true in the same cases), and moreover that any proposition formed by arbitrary conjunctions will have the same truth conditions, regardless of the location of the parentheses. This means that conjunction is associative, however, one should not assume that parentheses never serve a purpose. For instance, the sentence P ∧ (QR) does not have the same truth conditions of (PQ) ∨ R, so they are different sentences distinguished only by the parentheses. One can verify this by the truth-table method referenced above.

Note: For any arbitrary number of propositional constants, we can form a finite number of cases which list their possible truth-values. A simple way to generate this is by truth-tables, in which one writes Template:Mvar, Template:Mvar, ..., Template:Mvar, for any list of Template:Mvar propositional constants—that is to say, any list of propositional constants with Template:Mvar entries. Below this list, one writes 2k rows, and below Template:Mvar one fills in the first half of the rows with true (or T) and the second half with false (or F). Below Template:Mvar one fills in one-quarter of the rows with T, then one-quarter with F, then one-quarter with T and the last quarter with F. The next column alternates between true and false for each eighth of the rows, then sixteenths, and so on, until the last propositional constant varies between T and F for each row. This will give a complete listing of cases or truth-value assignments possible for those propositional constants.

Argument

The propositional calculus then defines an argument to be a set of propositions. A valid argument is a set of propositions, the last of which follows from—or is implied by—the rest. All other arguments are invalid. The simplest valid argument is modus ponens, one instance of which is the following set of propositions:

This is a set of three propositions, each line is a proposition, and the last follows from the rest. The first two lines are called premises, and the last line the conclusion. We say that any proposition Template:Mvar follows from any set of propositions , if Template:Mvar must be true whenever every member of the set is true. In the argument above, for any Template:Mvar and Template:Mvar, whenever PQ and Template:Mvar are true, necessarily Template:Mvar is true. Notice that, when Template:Mvar is true, we cannot consider cases 3 and 4 (from the truth table). When PQ is true, we cannot consider case 2. This leaves only case 1, in which Template:Mvar is also true. Thus Template:Mvar is implied by the premises.

This generalizes schematically. Thus, where Template:Mvar and Template:Mvar may be any propositions at all,

Other argument forms are convenient, but not necessary. Given a complete set of axioms (see below for one such set), modus ponens is sufficient to prove all other argument forms in propositional logic, thus they may be considered to be a derivative. Note, this is not true of the extension of propositional logic to other logics like first-order logic. First-order logic requires at least one additional rule of inference in order to obtain completeness.

The significance of argument in formal logic is that one may obtain new truths from established truths. In the first example above, given the two premises, the truth of Template:Mvar is not yet known or stated. After the argument is made, Template:Mvar is deduced. In this way, we define a deduction system to be a set of all propositions that may be deduced from another set of propositions. For instance, given the set of propositions , we can define a deduction system, Γ, which is the set of all propositions which follow from Template:Mvar. Reiteration is always assumed, so . Also, from the first element of Template:Mvar, last element, as well as modus ponens, Template:Mvar is a consequence, and so . Because we have not included sufficiently complete axioms, though, nothing else may be deduced. Thus, even though most deduction systems studied in propositional logic are able to deduce , this one is too weak to prove such a proposition.

Generic description of a propositional calculus

A propositional calculus is a formal system , where:

In this partition, is the set of operator symbols of arity Template:Mvar.
In the more familiar propositional calculi, Ω is typically partitioned as follows:
A frequently adopted convention treats the constant logical values as operators of arity zero, thus:
Some writers use the tilde (~), or N, instead of ¬; and some use the ampersand (&), the prefixed K, or instead of . Notation varies even more for the set of logical values, with symbols like {false, true}, {F, T}, or all being seen in various contexts instead of {0, 1}.

The language of , also known as its set of formulas, well-formed formulas, is inductively defined by the following rules:

  1. Base: Any element of the alpha set is a formula of .
  2. If are formulas and is in , then is a formula.
  3. Closed: Nothing else is a formula of .

Repeated applications of these rules permits the construction of complex formulas. For example:

  1. By rule 1, Template:Mvar is a formula.
  2. By rule 2, is a formula.
  3. By rule 1, Template:Mvar is a formula.
  4. By rule 2, is a formula.

Example 1. Simple axiom system

Let , where , , , are defined as follows:

Adopting negation and implication as the two primitive operations of a propositional calculus is tantamount to having the omega set partition as follows:

Example 2. Natural deduction system

Let , where , , , are defined as follows:

In the following example of a propositional calculus, the transformation rules are intended to be interpreted as the inference rules of a so-called natural deduction system. The particular system presented here has no initial points, which means that its interpretation for logical applications derives its theorems from an empty axiom set.

Our propositional calculus has ten inference rules. These rules allow us to derive other true formulas given a set of formulas that are assumed to be true. The first nine simply state that we can infer certain well-formed formulas from other well-formed formulas. The last rule however uses hypothetical reasoning in the sense that in the premise of the rule we temporarily assume an (unproven) hypothesis to be part of the set of inferred formulas to see if we can infer a certain other formula. Since the first nine rules don't do this they are usually described as non-hypothetical rules, and the last one as a hypothetical rule.

In describing the transformation rules, we may introduce a metalanguage symbol . It is basically a convenient shorthand for saying "infer that". The format is , in which Γ is a (possibly empty) set of formulas called premises, and Template:Mvar is a formula called conclusion. The transformation rule means that if every proposition in Γ is a theorem (or has the same truth value as the axioms), then Template:Mvar is also a theorem. Note that considering the following rule Conjunction introduction, we will know whenever Γ has more than one formula, we can always safely reduce it into one formula using conjunction. So for short, from that time on we may represent Γ as one formula instead of a set. Another omission for convenience is when Γ is an empty set, in which case Γ may not appear.

Negation introduction
From and , infer .
That is, .
Negation elimination
From , infer .
That is, .
Double negative elimination
From , infer Template:Mvar.
That is, .
Conjunction introduction
From Template:Mvar and Template:Mvar, infer .
That is, .
Conjunction elimination
From , infer Template:Mvar.
From , infer Template:Mvar.
That is, and .
Disjunction introduction
From Template:Mvar, infer .
From Template:Mvar, infer .
That is, and .
Disjunction elimination
From and and , infer Template:Mvar.
That is, .
Biconditional introduction
From and , infer .
That is, .
Biconditional elimination
From , infer .
From , infer .
That is, and .
Modus ponens (conditional elimination) 
From Template:Mvar and , infer Template:Mvar.
That is, .
Conditional proof (conditional introduction) 
From [accepting Template:Mvar allows a proof of Template:Mvar], infer .
That is, .

Basic and derived argument forms

Basic and Derived Argument Forms
Name Sequent Description
Modus Ponens If Template:Mvar then Template:Mvar; Template:Mvar; therefore Template:Mvar
Modus Tollens If Template:Mvar then Template:Mvar; not Template:Mvar; therefore not Template:Mvar
Hypothetical Syllogism If Template:Mvar then Template:Mvar; if Template:Mvar then Template:Mvar; therefore, if Template:Mvar then Template:Mvar
Disjunctive Syllogism Either Template:Mvar or Template:Mvar, or both; not Template:Mvar; therefore, Template:Mvar
Constructive Dilemma If Template:Mvar then Template:Mvar; and if Template:Mvar then Template:Mvar; but Template:Mvar or Template:Mvar; therefore Template:Mvar or Template:Mvar
Destructive Dilemma If Template:Mvar then Template:Mvar; and if Template:Mvar then Template:Mvar; but not Template:Mvar or not Template:Mvar; therefore not Template:Mvar or not Template:Mvar
Bidirectional Dilemma If Template:Mvar then Template:Mvar; and if Template:Mvar then Template:Mvar; but Template:Mvar or not Template:Mvar; therefore Template:Mvar or not Template:Mvar
Simplification Template:Mvar and Template:Mvar are true; therefore Template:Mvar is true
Conjunction Template:Mvar and Template:Mvar are true separately; therefore they are true conjointly
Addition Template:Mvar is true; therefore the disjunction (Template:Mvar or Template:Mvar) is true
Composition If Template:Mvar then Template:Mvar; and if Template:Mvar then Template:Mvar; therefore if Template:Mvar is true then Template:Mvar and Template:Mvar are true
De Morgan's Theorem (1) The negation of (Template:Mvar and Template:Mvar) is equiv. to (not Template:Mvar or not Template:Mvar)
De Morgan's Theorem (2) The negation of (Template:Mvar or Template:Mvar) is equiv. to (not Template:Mvar and not Template:Mvar)
Commutation (1) (Template:Mvar or Template:Mvar) is equiv. to (Template:Mvar or Template:Mvar)
Commutation (2) (Template:Mvar and Template:Mvar) is equiv. to (Template:Mvar and Template:Mvar)
Commutation (3) (Template:Mvar is equiv. to Template:Mvar) is equiv. to (Template:Mvar is equiv. to Template:Mvar)
Association (1) Template:Mvar or (Template:Mvar or Template:Mvar) is equiv. to (Template:Mvar or Template:Mvar) or Template:Mvar
Association (2) Template:Mvar and (Template:Mvar and Template:Mvar) is equiv. to (Template:Mvar and Template:Mvar) and Template:Mvar
Distribution (1) Template:Mvar and (Template:Mvar or Template:Mvar) is equiv. to (Template:Mvar and Template:Mvar) or (Template:Mvar and Template:Mvar)
Distribution (2) Template:Mvar or (Template:Mvar and Template:Mvar) is equiv. to (Template:Mvar or Template:Mvar) and (Template:Mvar or Template:Mvar)
Double Negation Template:Mvar is equivalent to the negation of not Template:Mvar
Transposition If Template:Mvar then Template:Mvar is equiv. to if not Template:Mvar then not Template:Mvar
Material Implication If Template:Mvar then Template:Mvar is equiv. to not Template:Mvar or Template:Mvar
Material Equivalence (1) (Template:Mvar iff Template:Mvar) is equiv. to (if Template:Mvar is true then Template:Mvar is true) and (if Template:Mvar is true then Template:Mvar is true)
Material Equivalence (2) (Template:Mvar iff Template:Mvar) is equiv. to either (Template:Mvar and Template:Mvar are true) or (both Template:Mvar and Template:Mvar are false)
Material Equivalence (3) (Template:Mvar iff Template:Mvar) is equiv to., both (Template:Mvar or not Template:Mvar is true) and (not Template:Mvar or Template:Mvar is true)
Exportation[9] from (if Template:Mvar and Template:Mvar are true then Template:Mvar is true) we can prove (if Template:Mvar is true then Template:Mvar is true, if Template:Mvar is true)
Importation If Template:Mvar then (if Template:Mvar then Template:Mvar) is equivalent to if Template:Mvar and Template:Mvar then Template:Mvar
Tautology (1) Template:Mvar is true is equiv. to Template:Mvar is true or Template:Mvar is true
Tautology (2) Template:Mvar is true is equiv. to Template:Mvar is true and Template:Mvar is true
Tertium non datur (Law of Excluded Middle) Template:Mvar or not Template:Mvar is true
Law of Non-Contradiction Template:Mvar and not Template:Mvar is false, is a true statement

Proofs in propositional calculus

One of the main uses of a propositional calculus, when interpreted for logical applications, is to determine relations of logical equivalence between propositional formulas. These relationships are determined by means of the available transformation rules, sequences of which are called derivations or proofs.

In the discussion to follow, a proof is presented as a sequence of numbered lines, with each line consisting of a single formula followed by a reason or justification for introducing that formula. Each premise of the argument, that is, an assumption introduced as an hypothesis of the argument, is listed at the beginning of the sequence and is marked as a "premise" in lieu of other justification. The conclusion is listed on the last line. A proof is complete if every line follows from the previous ones by the correct application of a transformation rule. (For a contrasting approach, see proof-trees).

Example of a proof

  • To be shown that AA.
  • One possible proof of this (which, though valid, happens to contain more steps than are necessary) may be arranged as follows:
Example of a Proof
Number Formula Reason
1 premise
2 From (1) by disjunction introduction
3 From (1) and (2) by conjunction introduction
4 From (3) by conjunction elimination
5 Summary of (1) through (4)
6 From (5) by conditional proof

Interpret as "Assuming Template:Mvar, infer Template:Mvar". Read as "Assuming nothing, infer that Template:Mvar implies Template:Mvar", or "It is a tautology that Template:Mvar implies Template:Mvar", or "It is always true that Template:Mvar implies Template:Mvar".

Soundness and completeness of the rules

The crucial properties of this set of rules are that they are sound and complete. Informally this means that the rules are correct and that no other rules are required. These claims can be made more formal as follows.

We define a truth assignment as a function that maps propositional variables to true or false. Informally such a truth assignment can be understood as the description of a possible state of affairs (or possible world) where certain statements are true and others are not. The semantics of formulas can then be formalized by defining for which "state of affairs" they are considered to be true, which is what is done by the following definition.

We define when such a truth assignment Template:Mvar satisfies a certain well-formed formula with the following rules:

With this definition we can now formalize what it means for a formula Template:Mvar to be implied by a certain set Template:Mvar of formulas. Informally this is true if in all worlds that are possible given the set of formulas Template:Mvar the formula Template:Mvar also holds. This leads to the following formal definition: We say that a set Template:Mvar of well-formed formulas semantically entails (or implies) a certain well-formed formua Template:Mvar if all truth assignments that satisfy all the formulas in Template:Mvar also satisfy Template:Mvar.

Finally we define syntactical entailment such that Template:Mvar is syntactically entailed by Template:Mvar if and only if we can derive it with the inference rules that were presented above in a finite number of steps. This allows us to formulate exactly what it means for the set of inference rules to be sound and complete:

Soundness: If the set of well-formed formulas Template:Mvar syntactically entails the well-formed formula Template:Mvar then Template:Mvar semantically entails Template:Mvar.

Completeness: If the set of well-formed formulas Template:Mvar semantically entails the well-formed formula Template:Mvar then Template:Mvar syntactically entails Template:Mvar.

For the above set of rules this is indeed the case.

Sketch of a soundness proof

(For most logical systems, this is the comparatively "simple" direction of proof)

Notational conventions: Let Template:Mvar be a variable ranging over sets of sentences. Let Template:Mvar and Template:Mvar range over sentences. For "Template:Mvar syntactically entails Template:Mvar" we write "Template:Mvar proves Template:Mvar". For "Template:Mvar semantically entails Template:Mvar" we write "Template:Mvar implies Template:Mvar".

We want to show: (A)(G) (if Template:Mvar proves Template:Mvar, then Template:Mvar implies Template:Mvar).

We note that "Template:Mvar proves Template:Mvar" has an inductive definition, and that gives us the immediate resources for demonstrating claims of the form "If Template:Mvar proves Template:Mvar, then ...". So our proof proceeds by induction.

Template:Ordered list Notice that Basis Step II can be omitted for natural deduction systems because they have no axioms. When used, Step II involves showing that each of the axioms is a (semantic) logical truth.

The Basis steps demonstrate that the simplest provable sentences from Template:Mvar are also implied by Template:Mvar, for any Template:Mvar. (The proof is simple, since the semantic fact that a set implies any of its members, is also trivial.) The Inductive step will systematically cover all the further sentences that might be provable—by considering each case where we might reach a logical conclusion using an inference rule—and shows that if a new sentence is provable, it is also logically implied. (For example, we might have a rule telling us that from "Template:Mvar" we can derive "Template:Mvar or Template:Mvar". In III.a We assume that if Template:Mvar is provable it is implied. We also know that if Template:Mvar is provable then "Template:Mvar or Template:Mvar" is provable. We have to show that then "Template:Mvar or Template:Mvar" too is implied. We do so by appeal to the semantic definition and the assumption we just made. Template:Mvar is provable from Template:Mvar, we assume. So it is also implied by Template:Mvar. So any semantic valuation making all of Template:Mvar true makes Template:Mvar true. But any valuation making Template:Mvar true makes "Template:Mvar or Template:Mvar" true, by the defined semantics for "or". So any valuation which makes all of Template:Mvar true makes "Template:Mvar or Template:Mvar" true. So "Template:Mvar or Template:Mvar" is implied.) Generally, the Inductive step will consist of a lengthy but simple case-by-case analysis of all the rules of inference, showing that each "preserves" semantic implication.

By the definition of provability, there are no sentences provable other than by being a member of Template:Mvar, an axiom, or following by a rule; so if all of those are semantically implied, the deduction calculus is sound.

Sketch of completeness proof

(This is usually the much harder direction of proof.)

We adopt the same notational conventions as above.

We want to show: If Template:Mvar implies Template:Mvar, then Template:Mvar proves Template:Mvar. We proceed by contraposition: We show instead that if Template:Mvar does not prove Template:Mvar then Template:Mvar does not imply Template:Mvar.

Template:Ordered list

QED

Another outline for a completeness proof

If a formula is a tautology, then there is a truth table for it which shows that each valuation yields the value true for the formula. Consider such a valuation. By mathematical induction on the length of the subformulas, show that the truth or falsity of the subformula follows from the truth or falsity (as appropriate for the valuation) of each propositional variable in the subformula. Then combine the lines of the truth table together two at a time by using "(Template:Mvar is true implies Template:Mvar) implies ((Template:Mvar is false implies Template:Mvar) implies Template:Mvar)". Keep repeating this until all dependencies on propositional variables have been eliminated. The result is that we have proved the given tautology. Since every tautology is provable, the logic is complete.

Interpretation of a truth-functional propositional calculus

An interpretation of a truth-functional propositional calculus is an assignment to each propositional symbol of of one or the other (but not both) of the truth values truth (T) and falsity (F), and an assignment to the connective symbols of of their usual truth-functional meanings. An interpretation of a truth-functional propositional calculus may also be expressed in terms of truth tables.[10]

For distinct propositional symbols there are distinct possible interpretations. For any particular symbol , for example, there are possible interpretations:

  1. is assigned T, or
  2. is assigned F.

For the pair , there are possible interpretations:

  1. both are assigned T,
  2. both are assigned F,
  3. is assigned T and is assigned F, or
  4. is assigned F and is assigned T.[10]

Since has , that is, denumerably many propositional symbols, there are , and therefore uncountably many distinct possible interpretations of .[10]

Interpretation of a sentence of truth-functional propositional logic

{{#invoke:main|main}}

If Template:Mvar and Template:Mvar are formulas of and is an interpretation of then:

means that Template:Mvar is logically valid

Some consequences of these definitions:

Alternative calculus

It is possible to define another version of propositional calculus, which defines most of the syntax of the logical operators by means of axioms, and which uses only one inference rule.

Axioms

Let Template:Mvar, Template:Mvar, and Template:Mvar stand for well-formed formulas. (The well-formed formulas themselves would not contain any Greek letters, but only capital Roman letters, connective operators, and parentheses.) Then the axioms are as follows:

Axioms
Name Axiom Schema Description
THEN-1 Add hypothesis Template:Mvar, implication introduction
THEN-2 Distribute hypothesis Template:Mvar over implication
AND-1 Eliminate conjunction
AND-2  
AND-3 Introduce conjunction
OR-1 Introduce disjunction
OR-2  
OR-3 Eliminate disjunction
NOT-1 Introduce negation
NOT-2 Eliminate negation
NOT-3 Excluded middle, classical logic
IFF-1 Eliminate equivalence
IFF-2  
IFF-3 Introduce equivalence
  • Axiom THEN-2 may be considered to be a "distributive property of implication with respect to implication."
  • Axioms AND-1 and AND-2 correspond to "conjunction elimination". The relation between AND-1 and AND-2 reflects the commutativity of the conjunction operator.
  • Axiom AND-3 corresponds to "conjunction introduction."
  • Axioms OR-1 and OR-2 correspond to "disjunction introduction." The relation between OR-1 and OR-2 reflects the commutativity of the disjunction operator.
  • Axiom NOT-1 corresponds to "reductio ad absurdum."
  • Axiom NOT-2 says that "anything can be deduced from a contradiction."
  • Axiom NOT-3 is called "tertium non datur" (Latin: "a third is not given") and reflects the semantic valuation of propositional formulas: a formula can have a truth-value of either true or false. There is no third truth-value, at least not in classical logic. Intuitionistic logicians do not accept the axiom NOT-3.

Inference rule

The inference rule is modus ponens:

.

Meta-inference rule

Let a demonstration be represented by a sequence, with hypotheses to the left of the turnstile and the conclusion to the right of the turnstile. Then the deduction theorem can be stated as follows:

If the sequence
has been demonstrated, then it is also possible to demonstrate the sequence
.

This deduction theorem (DT) is not itself formulated with propositional calculus: it is not a theorem of propositional calculus, but a theorem about propositional calculus. In this sense, it is a meta-theorem, comparable to theorems about the soundness or completeness of propositional calculus.

On the other hand, DT is so useful for simplifying the syntactical proof process that it can be considered and used as another inference rule, accompanying modus ponens. In this sense, DT corresponds to the natural conditional proof inference rule which is part of the first version of propositional calculus introduced in this article.

The converse of DT is also valid:

If the sequence
has been demonstrated, then it is also possible to demonstrate the sequence

in fact, the validity of the converse of DT is almost trivial compared to that of DT:

If
then
1:
2:
and from (1) and (2) can be deduced
3:
by means of modus ponens, Q.E.D.

The converse of DT has powerful implications: it can be used to convert an axiom into an inference rule. For example, the axiom AND-1,

can be transformed by means of the converse of the deduction theorem into the inference rule

which is conjunction elimination, one of the ten inference rules used in the first version (in this article) of the propositional calculus.

Example of a proof

The following is an example of a (syntactical) demonstration, involving only axioms THEN-1 and THEN-2:

Prove: (Reflexivity of implication).

Proof:

  1. Axiom THEN-2 with
  2. Axiom THEN-1 with
  3. From (1) and (2) by modus ponens.
  4. Axiom THEN-1 with
  5. From (3) and (4) by modus ponens.

Equivalence to equational logics

The preceding alternative calculus is an example of a Hilbert-style deduction system. In the case of propositional systems the axioms are terms built with logical connectives and the only inference rule is modus ponens. Equational logic as standardly used informally in high school algebra is a different kind of calculus from Hilbert systems. Its theorems are equations and its inference rules express the properties of equality, namely that it is a congruence on terms that admits substitution.

Classical propositional calculus as described above is equivalent to Boolean algebra, while intuitionistic propositional calculus is equivalent to Heyting algebra. The equivalence is shown by translation in each direction of the theorems of the respective systems. Theorems of classical or intuitionistic propositional calculus are translated as equations of Boolean or Heyting algebra respectively. Conversely theorems of Boolean or Heyting algebra are translated as theorems of classical or intuitionistic calculus respectively, for which is a standard abbreviation. In the case of Boolean algebra can also be translated as , but this translation is incorrect intuitionistically.

In both Boolean and Heyting algebra, inequality can be used in place of equality. The equality is expressible as a pair of inequalities and . Conversely the inequality is expressible as the equality , or as . The significance of inequality for Hilbert-style systems is that it corresponds to the latter's deduction or entailment symbol . An entailment

is translated in the inequality version of the algebraic framework as

Conversely the algebraic inequality is translated as the entailment

.

The difference between implication and inequality or entailment or is that the former is internal to the logic while the latter is external. Internal implication between two terms is another term of the same kind. Entailment as external implication between two terms expresses a metatruth outside the language of the logic, and is considered part of the metalanguage. Even when the logic under study is intuitionistic, entailment is ordinarily understood classically as two-valued: either the left side entails, or is less-or-equal to, the right side, or it is not.

Similar but more complex translations to and from algebraic logics are possible for natural deduction systems as described above and for the sequent calculus. The entailments of the latter can be interpreted as two-valued, but a more insightful interpretation is as a set, the elements of which can be understood as abstract proofs organized as the morphisms of a category. In this interpretation the cut rule of the sequent calculus corresponds to composition in the category. Boolean and Heyting algebras enter this picture as special categories having at most one morphism per homset, i.e., one proof per entailment, corresponding to the idea that existence of proofs is all that matters: any proof will do and there is no point in distinguishing them.

Graphical calculi

{{ safesubst:#invoke:Unsubst||$N=Unreferenced section |date=__DATE__ |$B= {{ safesubst:#invoke:Unsubst||$N=Unreferenced |date=__DATE__ |$B= {{#invoke:Message box|ambox}} }} }} It is possible to generalize the definition of a formal language from a set of finite sequences over a finite basis to include many other sets of mathematical structures, so long as they are built up by finitary means from finite materials. What's more, many of these families of formal structures are especially well-suited for use in logic.

For example, there are many families of graphs that are close enough analogues of formal languages that the concept of a calculus is quite easily and naturally extended to them. Indeed, many species of graphs arise as parse graphs in the syntactic analysis of the corresponding families of text structures. The exigencies of practical computation on formal languages frequently demand that text strings be converted into pointer structure renditions of parse graphs, simply as a matter of checking whether strings are well-formed formulas or not. Once this is done, there are many advantages to be gained from developing the graphical analogue of the calculus on strings. The mapping from strings to parse graphs is called parsing and the inverse mapping from parse graphs to strings is achieved by an operation that is called traversing the graph.

Other logical calculi

Propositional calculus is about the simplest kind of logical calculus in current use. It can be extended in several ways. (Aristotelian "syllogistic" calculus, which is largely supplanted in modern logic, is in some ways simpler – but in other ways more complex – than propositional calculus.) The most immediate way to develop a more complex logical calculus is to introduce rules that are sensitive to more fine-grained details of the sentences being used.

First-order logic (aka first-order predicate logic) results when the "atomic sentences" of propositional logic are broken up into terms, variables, predicates, and quantifiers, all keeping the rules of propositional logic with some new ones introduced. (For example, from "All dogs are mammals" we may infer "If Rover is a dog then Rover is a mammal".) With the tools of first-order logic it is possible to formulate a number of theories, either with explicit axioms or by rules of inference, that can themselves be treated as logical calculi. Arithmetic is the best known of these; others include set theory and mereology. Second-order logic and other higher-order logics are formal extensions of first-order logic. Thus, it makes sense to refer to propositional logic as "zeroth-order logic", when comparing it with these logics.

Modal logic also offers a variety of inferences that cannot be captured in propositional calculus. For example, from "Necessarily Template:Mvar" we may infer that Template:Mvar. From Template:Mvar we may infer "It is possible that Template:Mvar". The translation between modal logics and algebraic logics concerns classical and intuitionistic logics but with the introduction of a unary operator on Boolean or Heyting algebras, different from the Boolean operations, interpreting the possibility modality, and in the case of Heyting algebra a second operator interpreting necessity (for Boolean algebra this is redundant since necessity is the De Morgan dual of possibility). The first operator preserves 0 and disjunction while the second preserves 1 and conjunction.

Many-valued logics are those allowing sentences to have values other than true and false. (For example, neither and both are standard "extra values"; "continuum logic" allows each sentence to have any of an infinite number of "degrees of truth" between true and false.) These logics often require calculational devices quite distinct from propositional calculus. When the values form a Boolean algebra (which may have more than two or even infinitely many values), many-valued logic reduces to classical logic; many-valued logics are therefore only of independent interest when the values form an algebra that is not Boolean.

Solvers

Finding solutions to propositional logic formulas is an NP-complete problem. However, practical methods exist (e.g., DPLL algorithm, 1962; Chaff algorithm, 2001) that are very fast for many useful cases. Recent work has extended the SAT solver algorithms to work with propositions containing arithmetic expressions; these are the SMT solvers.

See also

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Higher logical levels

Related topics

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References

  1. Ancient Logic (Stanford Encyclopedia of Philosophy)
  2. {{#invoke:citation/CS1|citation |CitationClass=book }}
  3. Leibniz's Influence on 19th Century Logic
  4. {{#invoke:citation/CS1|citation |CitationClass=book }}
  5. Beth, Evert W.; "Semantic entailment and formal derivability", series: Mededlingen van de Koninklijke Nederlandse Akademie van Wetenschappen, Afdeling Letterkunde, Nieuwe Reeks, vol. 18, no. 13, Noord-Hollandsche Uitg. Mij., Amsterdam, 1955, pp. 309–42. Reprinted in Jaakko Intikka (ed.) The Philosophy of Mathematics, Oxford University Press, 1969
  6. 6.0 6.1 Truth in Frege
  7. 7.0 7.1 7.2 Russell's Use of Truth-Tables
  8. Wernick, William (1942) "Complete Sets of Logical Functions," Transactions of the American Mathematical Society 51, pp. 117–132.
  9. Template:Cite web
  10. 10.00 10.01 10.02 10.03 10.04 10.05 10.06 10.07 10.08 10.09 10.10 {{#invoke:citation/CS1|citation |CitationClass=book }}

Further reading

  • Brown, Frank Markham (2003), Boolean Reasoning: The Logic of Boolean Equations, 1st edition, Kluwer Academic Publishers, Norwell, MA. 2nd edition, Dover Publications, Mineola, NY.
  • Chang, C.C. and Keisler, H.J. (1973), Model Theory, North-Holland, Amsterdam, Netherlands.
  • Kohavi, Zvi (1978), Switching and Finite Automata Theory, 1st edition, McGraw–Hill, 1970. 2nd edition, McGraw–Hill, 1978.
  • Korfhage, Robert R. (1974), Discrete Computational Structures, Academic Press, New York, NY.
  • Lambek, J. and Scott, P.J. (1986), Introduction to Higher Order Categorical Logic, Cambridge University Press, Cambridge, UK.
  • Mendelson, Elliot (1964), Introduction to Mathematical Logic, D. Van Nostrand Company.

Related works

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External links

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