Propositional calculus

Propositional calculus is a branch of logic. It is also called propositional logic, statement logic, sentential calculus, sentential logic, or sometimes zeroth-order logic. It deals with propositions and relations between propositions, including the construction of arguments based on them. Compound propositions are formed by connecting propositions by logical connectives. Propositions that contain no logical connectives are called atomic propositions.
Unlike first-order logic, propositional logic does not deal with non-logical objects, predicates about them, or quantifiers. However, all the machinery of propositional logic is included in first-order logic and higher-order logics. In this sense, propositional logic is the foundation of first-order logic and higher-order logic.

Explanation

Logical connectives are found in natural languages. In English for example, some examples are "and", "or", "not" and "if".
The following is an example of a very simple inference within the scope of propositional logic:
Both premises and the conclusion are propositions. The premises are taken for granted and then with the application of modus ponens the conclusion follows.
As propositional logic is not concerned with the structure of propositions beyond the point where they can't be decomposed any more by logical connectives, this inference can be restated replacing those atomic statements with statement letters, which are interpreted as variables representing statements:
The same can be stated succinctly in the following way:
When is interpreted as "It's raining" and 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 axioms and inference rules 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.
In classical truth-functional propositional logic, formulas are interpreted as having precisely one of two possible truth values, the truth value of true or the truth value of false. The principle of bivalence and the law of excluded middle are upheld. Truth-functional propositional logic defined as such and systems isomorphic to it are considered to be zeroth-order logic. However, alternative propositional logics are possible. See Other logical calculi below.

History

Although propositional logic had been hinted by earlier philosophers, it was developed into a formal logic by Chrysippus in the 3rd century BC and expanded by his successor 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. Consequently, the system was essentially reinvented by Peter Abelard in the 12th century.
Propositional logic was eventually refined using symbolic logic. The 17th/18th-century mathematician 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 recreated by logicians like George Boole and Augustus De Morgan completely independent of Leibniz.
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." 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. The invention of truth-tables, however, is of uncertain attribution.
Within works by Frege and Bertrand Russell, are ideas influential to the invention of truth tables. The actual tabular structure, itself, is generally credited to either Ludwig Wittgenstein or Emil Post. 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 Jan Łukasiewicz, Ernst Schröder, Alfred North Whitehead, William Stanley Jevons, John Venn, and Clarence Irving Lewis. 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.".

Terminology

In general terms, a calculus is a formal system that consists of a set of syntactic expressions, a distinguished subset of these expressions, plus a set of formal rules that define a specific binary relation, intended to be interpreted as 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 as statements, and the rules, known to be inference rules, are typically intended to be truth-preserving. In this setting, the rules can then be used to derive formulas representing true statements from given formulas representing true statements.
The set of axioms may be empty, a nonempty finite set, or a countably infinite set. 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.
The language of a propositional calculus consists of

a set of primitive symbols, variously referred to as atomic formulas, placeholders, proposition letters, or variables, and
a set of operator symbols, variously interpreted as 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,, and, propositional variables by,, and, and schematic letters are often Greek letters, most often,, and.

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:

their language, that is, the particular collection of primitive symbols and operator symbols,
the set of axioms, or distinguished formulas, and
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,. All propositions require exactly one of two truth-values: true or false. For example, let be the proposition that it is raining outside. This will be true if it is raining outside and false otherwise.

We then define truth-functional operators, beginning with negation. represents the negation of, which can be thought of as the denial of. In the example above, expresses that it is not raining outside, or by a more standard reading: "It is not the case that it is raining outside." When is true, is false; and when is false, is true. always has the same truth-value as.
Conjunction is a truth-functional connective which forms a proposition out of two simpler propositions, for example, and. The conjunction of and is written, and expresses that each are true. We read as " and ". For any two propositions, there are four possible assignments of truth values:
# is true and is true
# is true and is false
# is false and is true
# is false and is false
Disjunction resembles conjunction in that it forms a proposition out of two simpler propositions. We write it, and it is read " or ". It expresses that either or is true. Thus, in the cases listed above, the disjunction of with is true in all cases except case 4. Using the example above, the disjunction expresses that it is either raining outside or there is a cold front over Kansas.
Material conditional also joins two simpler propositions, and we write, which is read "if then ". The proposition to the left of the arrow is called the antecedent and the proposition to the right is called the consequent. It expresses that is true whenever is true. Thus is true in every case above except case 2, because this is the only case when is true but is not. Using the example, if then 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, which is read " if and only if ". It expresses that and have the same truth-value, and so, in cases 1 and 4, is true if and only if is true, 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, is also a proposition. Likewise, for any propositions and, is a proposition, and similarly for disjunction, conditional, and biconditional. This implies that, for instance, 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, is not a well-formed formula, because we do not know if we are conjoining with or if we are conjoining with. Thus we must write either to represent the former, or to represent the latter. By evaluating the truth conditions, we see that both expressions have the same truth conditions, 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 does not have the same truth conditions of, 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,,...,, for any list of propositional constants—that is to say, any list of propositional constants with entries. Below this list, one writes rows, and below one fills in the first half of the rows with true and the second half with false. Below 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 list of propositions. A valid argument is a list 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 list of propositions:
This is a list 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 follows from any set of propositions, if must be true whenever every member of the set is true. In the argument above, for any and, whenever and are true, necessarily is true. Notice that, when is true, we cannot consider cases 3 and 4. When is true, we cannot consider case 2. This leaves only case 1, in which is also true. Thus is implied by the premises.
This generalizes schematically. Thus, where and may be any propositions at all,
Other argument forms are convenient, but not necessary. Given a complete set of axioms, 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 is not yet known or stated. After the argument is made, 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,, which is the set of all propositions which follow from. Reiteration is always assumed, so. Also, from the first element of, last element, as well as modus ponens, 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:

The alpha set is a countably infinite set of elements called proposition symbols or propositional variables. Syntactically speaking, these are the most basic elements of the formal language, otherwise referred to as atomic formulas or terminal elements. In the examples to follow, the elements of are typically the letters,,, and so on.
The omega set is a finite set of elements called operator symbols or logical connectives. The set is partitioned into disjoint subsets as follows:
The zeta set is a finite set of transformation rules that are called inference rules when they acquire logical applications.
The iota set is a countable set of initial points that are called axioms when they receive logical interpretations.

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

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

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

By rule 1, is a formula.
By rule 2, is a formula.
By rule 1, is a formula.
By rule 2, is a formula.
Example 1. Simple axiom system

Let, where,,, are defined as follows:

The alpha set, is a countably infinite set of symbols, for example:
Of the three connectives for conjunction, disjunction, and implication, one can be taken as primitive and the other two can be defined in terms of it and negation. Indeed, all of the logical connectives can be defined in terms of a sole sufficient operator. The biconditional can of course be defined in terms of conjunction and implication, with defined as.
An axiom system proposed by Jan Łukasiewicz, and used as the propositional-calculus part of a Hilbert system, formulates a propositional calculus in this language as follows. The axioms are all substitution instances of:
The rule of inference is modus ponens. Then is defined as, and is defined as. This system is used in Metamath formal proof database.
Example 2. Natural deduction system

Let, where,,, are defined as follows:

The alpha set, is a countably infinite set of symbols, for example:
:
The omega set partitions 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.

The set of initial points is empty, that is,.
The set of transformation rules,, is described as follows:

Our propositional calculus has eleven inference rules. These rules allow us to derive other true formulas given a set of formulas that are assumed to be true. The first ten 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 hypothesis to be part of the set of inferred formulas to see if we can infer a certain other formula. Since the first ten 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 is a set of formulas called premises, and is a formula called conclusion. The transformation rule means that if every proposition in is a theorem, then is also a theorem. Note that considering the following rule Conjunction introduction, we will know whenever 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 as one formula instead of a set. Another omission for convenience is when is an empty set, in which case may not appear.
; Negation introduction: From and, infer.
; Negation elimination: From, infer.
; Double negation elimination: From, infer.
; Conjunction introduction: From and, infer.
; Conjunction elimination: From, infer.
; Disjunction introduction: From, infer.
; Disjunction elimination: From and and, infer.
; Biconditional introduction: From and, infer.
;Biconditional elimination: From, infer.
;Modus ponens : From and, infer.
; Conditional proof : From , infer.

Basic and derived argument forms

Name	Sequent	Description
Modus Ponens		If then ; ; therefore
Modus Tollens		If then ; not ; therefore not
Hypothetical Syllogism		If then ; if then ; therefore, if then
Disjunctive Syllogism		Either or, or both; not ; therefore,
Constructive Dilemma		If then ; and if then ; but or ; therefore or
Destructive Dilemma		If then ; and if then ; but not or not ; therefore not or not
Bidirectional Dilemma		If then ; and if then ; but or not ; therefore or not
Simplification		and are true; therefore is true
Conjunction		and are true separately; therefore they are true conjointly
Addition		is true; therefore the disjunction is true
Composition		If then ; and if then ; therefore if is true then and are true
De Morgan's Theorem		The negation of is equiv. to
De Morgan's Theorem		The negation of is equiv. to
Commutation		is equiv. to
Commutation		is equiv. to
Commutation		is equiv. to
Association		or is equiv. to or
Association		and is equiv. to and
Distribution		and is equiv. to or
Distribution		or is equiv. to and
Double Negation	and	is equivalent to the negation of not
Transposition		If then is equiv. to if not then not
Material Implication		If then is equiv. to not or
Material Equivalence		is equiv. to and
Material Equivalence		is equiv. to either or
Material Equivalence		is equiv to., both and
Exportation		from we can prove
Importation		If then is equivalent to if and then
Tautology		is true is equiv. to is true or is true
Tautology		is true is equiv. to is true and is true
Tertium non datur		or not is true
Law of Non-Contradiction		and not 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..

Example of a proof in natural deduction system

To be shown that.
One possible proof of this may be arranged as follows:

Number	Formula	Reason
		premise
		From by disjunction introduction
		From and by conjunction introduction
		From by conjunction elimination
		Summary of through
		From by conditional proof

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

Example of a proof in a classical propositional calculus system

We now prove the same theorem in the axiomatic system by Jan Łukasiewicz described above, which is an example of a classical propositional calculus systems, or a Hilbert-style deductive system for propositional calculus.
The axioms are:
And the proof is as follows:

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.
Note that the proofs for the soundness and completeness of the propositional logic are not themselves proofs in propositional logic ; these are theorems in ZFC used as a metatheory to prove properties of propositional logic.
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 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 satisfies a certain well-formed formula with the following rules:

satisfies the propositional variable if and only if
satisfies if and only if does not satisfy
satisfies if and only if satisfies both and
satisfies if and only if satisfies at least one of either or
satisfies if and only if it is not the case that satisfies but not
satisfies if and only if satisfies both and or satisfies neither one of them

With this definition we can now formalize what it means for a formula to be implied by a certain set of formulas. Informally this is true if in all worlds that are possible given the set of formulas the formula also holds. This leads to the following formal definition: We say that a set of well-formed formulas semantically entails a certain well-formed formula if all truth assignments that satisfy all the formulas in also satisfy.
Finally we define syntactical entailment such that is syntactically entailed by 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 syntactically entails the well-formed formula then semantically entails.
Completeness: If the set of well-formed formulas semantically entails the well-formed formula then syntactically entails.
For the above set of rules this is indeed the case.

Sketch of a soundness proof

Notational conventions: Let be a variable ranging over sets of sentences. Let and range over sentences. For " syntactically entails " we write " proves ". For " semantically entails " we write " implies ".
We want to show: .
We note that " proves " has an inductive definition, and that gives us the immediate resources for demonstrating claims of the form "If proves, then...". So our proof proceeds by induction.
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 logical truth.
The Basis steps demonstrate that the simplest provable sentences from are also implied by, for any. 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. 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, an axiom, or following by a rule; so if all of those are semantically implied, the deduction calculus is sound.

Sketch of completeness proof

We adopt the same notational conventions as above.
We want to show: If implies, then proves. We proceed by contraposition: We show instead that if does not prove then does not imply. If we show that there is a model where does not hold despite being true, then obviously does not imply. The idea is to build such a model out of our very assumption that does not prove.
Thus every system that has modus ponens as an inference rule, and proves the following theorems is complete:

p →
→
p →
p →
¬p →
p → p
p →
→ → )

The first five are used for the satisfaction of the five conditions in stage III above, and the last three for proving the deduction theorem.

Example

As an example, it can be shown that as any other tautology, the three axioms of the classical propositional calculus system described earlier can be proven in any system that satisfies the above, namely that has modus ponens as an inference rule, and proves the above eight theorems. Indeed, out of the eight theorems, the last two are two of the three axioms; the third axiom,, can be proven as well, as we now show.
For the proof we may use the hypothetical syllogism theorem, since it only relies on the two axioms that are already in the above set of eight theorems.
The proof then is as follows:

Verifying completeness for the classical propositional calculus system

We now verify that the classical propositional calculus system described earlier can indeed prove the required eight theorems mentioned above. We use several lemmas proven here:
We also use the method of the hypothetical syllogism metatheorem as a shorthand for several proof steps.

p → - proof:
→ - proof:
p → - proof:
p → - proof:
¬p → - proof:
p → p - proof given in the proof example above
p → - axiom
→ → ) - axiom
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 of each propositional variable in the subformula. Then combine the lines of the truth table together two at a time by using " implies ". 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 of the truth values truth and falsity, 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.
For distinct propositional symbols there are distinct possible interpretations. For any particular symbol, for example, there are possible interpretations:

is assigned T, or
is assigned F.

For the pair, there are possible interpretations:

both are assigned T,
both are assigned F,
is assigned T and is assigned F, or
is assigned F and is assigned T.

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

Interpretation of a sentence of truth-functional propositional logic

If and are formulas of and is an interpretation of then the following definitions apply:

A sentence of propositional logic is true under an interpretation if assigns the truth value T to that sentence. If a sentence is true under an interpretation, then that interpretation is called a model of that sentence.
is false under an interpretation if is not true under.
A sentence of propositional logic is logically valid if it is true under every interpretation.
A sentence of propositional logic is a semantic consequence of a sentence if there is no interpretation under which is true and is false.
A sentence of propositional logic is consistent if it is true under at least one interpretation. It is inconsistent if it is not consistent.

Some consequences of these definitions:

For any given interpretation a given formula is either true or false.
No formula is both true and false under the same interpretation.
is false for a given interpretation is true for that interpretation; and is true under an interpretation is false under that interpretation.
If and are both true under a given interpretation, then is true under that interpretation.
If and, then.
is true under is not true under.
is true under either is not true under or is true under.
A sentence of propositional logic is a semantic consequence of a sentence is logically valid, that is, .
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,, and stand for well-formed formulas. Then the axioms are as follows:

Name	Axiom Schema	Description
		Add hypothesis, implication introduction
		Distribute hypothesis over implication
		Eliminate conjunction

		Introduce conjunction
		Introduce disjunction

		Eliminate disjunction
		Introduce negation
		Eliminate negation
		Excluded middle, classical logic
		Eliminate equivalence

		Introduce equivalence

Axiom may be considered to be a "distributive property of implication with respect to implication."
Axioms and correspond to "conjunction elimination". The relation between and reflects the commutativity of the conjunction operator.
Axiom corresponds to "conjunction introduction."
Axioms and correspond to "disjunction introduction." The relation between and reflects the commutativity of the disjunction operator.
Axiom corresponds to "reductio ad absurdum."
Axiom says that "anything can be deduced from a contradiction."
Axiom is called "tertium non datur" 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.
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:
This deduction theorem 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:
in fact, the validity of the converse of DT is almost trivial compared to that of DT:
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 of the propositional calculus.

Example of a proof

The following is an example of a demonstration, involving only axioms and :
Prove: .
Proof:

: Axiom with
: Axiom with
: From and by modus ponens.
: Axiom with
: From and 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

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. 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 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. 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 " we may infer that. From we may infer "It is possible that ". 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. 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. These logics often require calculational devices quite distinct from propositional calculus. When the values form a Boolean algebra, 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 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.

Higher logical levels

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