Logic Terms and Concepts
conceptual map FOL

Map of the basic concepts of logic

Table of Contents
 atomic sentence 
 bearer 
 bind 
 bound variable 
 closed sentence 
 compact 
 compactness theorem 
 complete 
 completeness theorem 
 conjunction 
 consistent 
 denotation 
 designation 
 disjunction 
 domain 
 dot 
 elementary statement 
 existential quantification 
 existential quantifier 
 first-order logic 
 formula 
 free variable 
 function 
 Gödel's first incompleteness theorem 
 Gödel's first inconsistency theorem 
 identity 
 iff 
 implication 
 inconsistent 
 interpretation 
 invalid 
 language 
 logical equivalence 
 logical symbol 
 material biconditional 
 material conditional 
 material implication 
 meta-logical 
 metavariable 
 model 
 name 
 negation 
 non-logical symbol 
 open sentence 
 paradoxes of classical logic 
 parenthesization 
 predicate 
 propositional variable 
 quantification 
 quantifier 
 reference 
 satisfiable 
 schema 
 second-order logic 
 sentence 
 sentence letter 
 Skolem-Löwenheim theorem 
 sound 
 statement 
 statement letter 
 structure 
 tautology 
 term 
 theory 
 truth table 
 truth value 
 truth-functional 
 universal quantification 
 universal quantifier 
 unsatisfiable 
 valid 
 variable 
 well-formed formula 

Speaking in “logic”

Pronouncing logic is as easy as αβγ.  Just follow the examples below. 

Pronouncing logic
Logic Pronunciation
true true
true
false
F false
ff false
tt true
a A
¬ b not B
d ∧ ⊤ D and true
c ∨ ⊥ C or false
∀x (x = x) For every x, x equals x
∃y (y ≠ y) For some y, y does not equal y
S is valid
R ⊢ S R implies S
R ⇔ S R is logically equivalent to S

Logical and meta-logical

It is important to distinguish logical statements, operators, and relationships from meta-logical statements, operators, and relationships.  Logical statements are those that state something about objects in the domain of discourse, or something about a logical relationship between sentence variables.  Meta-logical statements, on the other hand, say something about one or more logical statements.  Examples: “A ∧ B” and “¬ (¬ A ∨ ¬ B)” are logical statements; “ ‘A ∧ B is logically equivalent to ‘¬ (¬ A ∨ ¬ B)’ ” is a meta-logical statement. 

Quine characterizes this as a distinction between mentioning and using [Quine1982-ml p.50], or perhaps more vigorously as a difference between stating and naming.  When I state that A ∧ B and that ¬ (¬ A ∨ ¬ B), I have used those two logical statements, not mentioned them.  When I state that “A ∧ B is logically equivalent to ¬ (¬ A ∨ ¬ B)”, on the other hand, I have named the two statements (by presenting them in quotes), not used them, and I have said something about the statements (by naming them and stating a relationship). 

The authors whose work is collated here keep this distinction in part by being quite specific about what the syntactic form of a logical statement is, and thus excluding all other statements.  Thus all meta-logical statements can be syntactically distinguished from logical statements, if one follows the rules fastidiously.  See formula, sentence in the glossary below. 

Summary of presentations of logic

This section summarizes in tabular form some of the notation and usage for the authors discussed here. 

Logical symbols
Citation true false not and or if iff univ. exist. ident.
[Bell+DeVidi+Solomon2001-lo] ¬
[Barwise+Etchemendy1992-lfol] TRUE FALSE ¬ ∀x ∃x =
[Boolos+Jeffrey1989-cl] 1 0 - & v ∀x ∃x =
[Kleene1967-ml] t ƒ ¬ & ∀x ∃x
[Quine1982-ml] p, - pq
(Hilbert) &
[Whitehead+Russell1910-13-pm] ~ (x) (∃x)
(Peano) -
(Pierce) p
Notable variations from current logical syntax
Description Example Citations
Negation by overbar p Quine1982-ml, (Pierce)
Conjunction by catenation ab Quine1982-ml
Grouping by dots a.b∨ c Quine1982-ml, Whitehead+Russell1910-13-pm, (Peano)
Logical terminology
[Boolos+Jeffrey1989-cl] [Barwise+Etchemendy1992-lfol] [Quine1982-ml] [Bell+DeVidi+Solomon2001-lo]
not negation
and conjunction
or disjunction
if ... then material implication implication
iff material biconditional biconditional
Meta-logical symbols
Citation implies valid equivalent interpretation
[Boolos+Jeffrey1989-cl] I
[Barwise+Etchemendy1992-lfol]
General non-logical symbol usage
Citation name function sentence predicate
[Boolos+Jeffrey1989-cl] a b c ... f(a) S xLy xFy ...
[Barwise+Etchemendy1992-lfol] a b c ... ab(a) bc(a,b) ... A(a) B(a,b) ...
[Quine1982-ml] Fx Gy ... 

Glossary of basic terminology

atomic sentence
Any of the following [Boolos+Jeffrey1989-cl p.102]: An atomic sentence has no component sentences or formulae; the components of = and other predicates are term.  Boolos and Jeffrey distinguish atomic sentences, for which there are no corresponding formulae, from other sentences, which correspond to formulae but have no free variables

See sentence

bearer
See designation
bind
A quantifier is said to bind the variable that is its parameter [Barwise+Etchemendy1992-lfol p.115].  Free occurrences of the variable in the subformula of the quantification are bound by the quantifier. 
bound variable
A bound variable is one whose relation to the objects of the domain is specified.  In first-order logic, variables are bound by quantifiers; any variable that is not bound by a quantifier is a free variable whose relation to the objects of the domain is unknown. 

In a formula ∀x F or ∃x F, the variable x is bound. 

See free variable, quantifier, variable

closed sentence
See sentence
compact
A logic is compact if for any countable set of sentences Δ, there is a finite subset δ of Δ such that δ is unsatisfiable iff Δ is unsatisfiable. 
compactness theorem
(Of first-order logic.)  An enumerable set θ of sentences is unsatisfiable if and only if there is a finite subset of θ that is unsatisfiable [Boolos+Jeffrey1989-cl p.140]. 
complete
A theory T is complete if for every possible sentence A in the language of T, either A or ¬A (or both) is a theorem of T [Boolos+Jeffrey1989-cl p.177]. 

Informally, a formal system is complete if every true statement can be derived in it. 

A theory is complete and consistent if for every possible sentence A in the language of T, exactly one of A or ¬A is a theorem of T [Boolos+Jeffrey1989-cl p.177]. 

Compare consistent.  See Gödel's first inconsistency theorem.

completeness theorem
(Of first-order logic.)  If an enumerable set Δ of sentences is unsatisfiable, then there is a finite derivation from Δ containing a contradiction [Boolos+Jeffrey1989-cl p.131]. 
conjunction
A conjunction is a formula composed of two subformulae; the conjunction is true if and only if both its subformulae are true. 
Truth table for conjunction
α β α ∧ β
true true true
true false false
false true false
false false false

A conjunction is frequently written as α ∧ β.  Some other notations are listed in the table of logical symbols

A useful mnemonic for is that it resembles the ‘A’ in “and”. 

consistent
A theory is consistent if there is no theorem of the theory whose negation is also a theorem [Boolos+Jeffrey1989-cl p.173]. 

Informally, a system is consistent if there is no sentence A for which it is possible to derive both A and ¬A in the system. 

See satisfiable

denotation
See designation
designation
For a name, the object in the domain that the name designates [Boolos+Jeffrey1989-cl p.99]. 

Synonyms:  bearer, denotation, reference

disjunction
A disjunction is a formula composed of two subformulae; the disjunction is false if and only if both its subformulae are false. 
Truth table for disjunction
α β α ∨ β
true true true
true false true
false true true
false false false

A disjunction is frequently written as α ∨ β.  Some other notations are listed in the table of logical symbols

The symbol is not an arbitrary choice, but rather derives from the Latin vel whose meaning corresponds to disjunction [Quine1982-ml p.12].  (Latin also possesses a word aut for exclusive or – A or B but not both.)

domain
A collection of entities (about which we wish to reason). 

Example: in logic, a domain that is frequently of interest is the domain of the integers and their operations and relations. 

Also called universe, universe of discourse

dot
(Deprecated.)  Quine [Quine1982-ml] sometimes uses a system of dots to express grouping, instead of parentheses.  Where parentheses enclose groups, dots divide groups; where parentheses nest to indicate deeper grouping, dots are doubled (or, presumably, tripled or stacked even higher) to divide still larger groups.  This notation is obsolete. 

Example:

s ∨ (p ∧ (q → r) ↔ (p ∨ q) ∧ r) ∧ t
would be “dotted” as
s ∨ : p ∧ . q → r . ↔ . p ∨ q . ∧ r : ∧ t
[Quine1982-ml p.30, written with for conjunction]. 

Church uses a single dot to mean the same as a left bracket at that location and a right bracket at the end of the sentence [Church1958-iml p. 42]. 

elementary statement
See sentence letter.  [Bell+DeVidi+Solomon2001-lo p.6-7]. 
existential quantification
A formula constructed from a variable x (the parameter of the quantification) and a subformula F.  The existential quantification is true if and only if the subformula F is true for at least one possible value of the variable x (some object in the domain). 

An existential quantification is written as ∃x F; occasionally the notation ∃x.F is seen, but it is obsolete (see dot). 

The quantifier ∃x is said to bind x (see bound variable, free variable).  Any unbound instance of x in F becomes bound in ∃x (F)

See existential quantifier, quantification, universal quantification

existential quantifier
∃x for some variable x. 
first-order logic
A logic in which quantification is restricted to domain objects [Bell+DeVidi+Solomon2001-lo p. 122]. 

See second-order logic

formula
(Plural formulae or formulas.)

A syntactically-correct utterance in a logical language that may contains zero or more free variables

Utterances with precisely zero free variables are also called sentences

For Boolos and Jeffrey, a formula is one of the following, where α, and β are also formulae [Boolos+Jeffrey1989-cl p.101]:

Formulae
1. (negation)
2. (α&β) (conjunction)
3. vβ) (disjunction)
4. (α→ β) (material implication)
5. (α↔ β) (material biconditional)
6. ∀v α (universal)
7. ∃v α (existential)

In addition, any atomic sentence is a formula. 

Barwise and Etchemendy define formulae similarly, but using their own logical symbols [Barwise+Etchemendy1992-lfol p.117]. 

Quine uses open sentence for this concept [Quine1982-ml p.134]. 

Compare sentence

free variable
A variable in a formula F is said to be free (or unbound) in F if it is not a bound variable in F

A free variable is one whose relation to the objects in the domain is unknown. 

See bound variable, variable

function
A function takes one or more terms and produces a term

Compare predicate which takes one or more domain objects and produces either true or false. 

Gödel's first incompleteness theorem
There is no consistent, complete, axiomatizable extension of Q (a specific axiomatization of the natural numbers) [Boolos+Jeffrey1989-cl p.179]. 

Informally, the proof uses what is termed the Gödel statement, G, which is "G cannot be proven true."  If G can be proven under the extension of Q, then the extension would be inconsistent (as G states that it can't be proven);  and if G can't be proven, then it is true, and the extension is incomplete. 

Formally, the proof proceeds by representing any possible statement in the language by a number, so that (after a number of intermediate stages) provability of statements reduces to decidability of the set of numbers corresponding to them.  (I was told by my logic professor that only a few dozen people in the world, including himself, really understand this proof in its entirety.) 

Gödel's first inconsistency theorem
identity
The identity predicate is a binary predicate that is true only for two references to the same object.  We write R1 = R2, where R1 and R2 are names or functions [Boolos+Jeffrey1989-cl p.96]. 

Compare logical equivalence which is a meta-logical comparison of two sentences. 

iff
An abbreviation for “if and only if”. 

See logical equivalence, material biconditional

implication
Sentence S1 implies sentence S2 if for every interpretation I that is an interpretation of both sentences, S2 is true in I if S1 is true in I [Boolos+Jeffrey1989-cl p.104]. 

If sentence S1 implies sentence S2, we write S1 ⊢ S2 [Boolos+Jeffrey1989-cl p.104]. 

Implication can be generalized to sets ΓS1,S2,...,Sn} of sentences: Γ implies sentence Sn+1 if for every I that is an interpretation of all n+1 sentences, Sn+1 is true in I if each of S1,S2,...,Sn is true in I

Implication is a meta-logical operation, not to be confused with the logical formula of material implication

inconsistent
See unsatisfiable
interpretation
An interpretation gives meaning to the non-logical symbols of a language.  Boolos and Jeffrey define an interpretation as [Boolos+Jeffrey1989-cl p.98-99]:
  1. A domain that is a non-empty set of objects that the variables of the language range over.  Synonyms: universe, universe of discourse
  2. A designation for each name in the language; the designation is an entity in the domain.  Synonyms: bearer, denotation, reference
  3. A function for each function symbol in the language.  The function for an n-place function symbol maps every n-tuple of domain objects to an object. 
  4. A truth-value for each sentence letter in the language (true or false). 
  5. A characteristic function for each predicate letter in the language.  The characteristic function for an n-place predicate letter maps every n-tuple of domain objects to either true or false. 

An interpretation may also be viewed as a operator that takes a sentence and gives either true or false.  Boolos and Jeffrey write this operation on sentence S as I(S) [Boolos+Jeffrey1989-cl p.101]. 

Interpretations are also known as models (but see glossary entry for model) or interpretation

The domain and range of logical and non-logical symbols
Symbol Domain and range
variable objects in the domain
name a specific object in the domain
function symbol n-tuple of domain objects domain object
predicate letter n-tuple of domain objects true or false
sentence letter true or false

See model

invalid
A sentence is invalid if it is false in every interpretation.

See satisfiable, unsatisfiable, valid

language
Boolos and Jeffrey use language to refer to a collection of non-logical symbols which, together with the usual logical symbols, may be used to construct formulae and sentences about a particular domain [Boolos+Jeffrey1989-cl p.97]. 
logical equivalence
Two sentences S1 and S2 are logically equivalent if for every I that is an interpretation of both S1 and S2, I(S1) = I(S2) [Boolos+Jeffrey1989-cl p.107, Quine1982-ml p.60]. 

Logical equivalence is a meta-logical relationship, not to be confused with the logical operation material biconditional

logical symbol
Any of the connectives, quantifiers, and other symbols common to all first-order logic languages; For Boolos and Jeffrey these are the variables (enumerably infinitely many) and
- & v = ( ) ,
[Boolos+Jeffrey1989-cl p.97].  Compare non-logical symbol.  The table of logical symbols lists the logical symbols (other than variables) used by various authors. 

Various authors use different terminology for the logical operators, summarized in the table of logical terminology

Various authors also use different rules for parenthesization

material biconditional
(Deprecated.)  A material biconditional is a formula composed of two subformulae;  the material biconditional is true if and only if both subformulae are true or both subformulae are false.  It is sometimes described as the logical expression of “α if and only if β”;  however, usually logical equivalence is what is meant. 
Truth table for material biconditional
α β α ↔ β
true true true
true false false
false true false
false false true

A material biconditional is almost universally written as α ↔ β.  Other notations are listed in the table of logical symbols

α ↔ β is logically equivalent to (α ∧ β) ∨ (¬ α ∧ ¬ β), and is probably best considered to be an abbreviation for it. 

The material biconditional is a logical formula, not to be confused with the meta-logical relation of logical equivalence.  Perhaps because of this potential confusion, the material biconditional does not seem to be used as commonly now as in the past. 

material conditional
See material implication
material implication
(Deprecated.)  A material implication is a formula composed of two subformulae, called its antecedent and consequent; the antecedent is the subformula on the left.  The material implication is true if and only if its consequent is true or its antecedent is false.  It is sometimes described as the logical expression of “if α then β”;  however, usually implication is closer to what is meant. 
Truth table for material implication
α β α → β
true true true
true false false
false true true
false false true

A material implication is frequently written as α→β.  Some other notations are listed in the table of logical symbols

αβ is logically equivalent to (¬ α) ∨ β, and is probably best considered as an abbreviation for it.  Many authors have noted that a material implication seems to be making a statement about one thing causing another (“if α then β”), but in fact doesn't have anything to do with causality.  See paradoxes of classical logic

The material implication is a logical formula, not to be confused with the meta-logical relation of implication.  Perhaps because of this potential confusion and the recognition that material implication isn't appropriate for describing cause and effect, the material implication does not seem to be used as commonly now. 

Also known as conditional

meta-logical
Formulae and sentences express properties of domain objects, or make statements about sentence variables and truth values; meta-logical utterances express properties of or refer to formulae and sentences. 

The meta-logical operators and properties include implication, satisfiability validity, logical equivalence, and interpretation.  See also metavariable

metavariable
A metavariable is a variable (in the general sense) that refers to a formula [Epstein2001-pl p.13]. 

Compare variable; sentence letter, name

model
A model of a sentence is an interpretation in which that sentence is true [Boolos+Jeffrey1989-cl p.105]. 
name
A name designates to a particular object in the domain of the language [Boolos+Jeffrey1989-cl p.98-99, Barwise+Etchemendy1992-lfol p.10]. 

It is possible for an object to have more than one name; but each name refers to only one object. 

See non-logical symbol, interpretation

negation
A negation is a formula composed of a single subformulae; the negation is true if and only if the subformula is false. 
Truth table for negation
α ¬ α
true false
false true

A negation is frequently written as ¬ α.  Some other notations are listed in the table of logical symbols

non-logical symbol
Any of the symbols of a particular logic language.  Boolos and Jeffrey divide these into names, function symbols, sentence letters, and predicate letters (and appropriate the term language to mean a set of non-logical symbols) [Boolos+Jeffrey1989-cl p.97].  See interpretation

Of course, each language may have its own conventions for the use of its symbols; the language of arithmetic uses its own functions and predicates 2+3×4, 5^2, and 6<4, as does set theory a ∈ b and T ∪ U.  General, non-specific usage for various texts is summarized in the table of non-logical syntax

open sentence
See formula
paradoxes of classical logic
Material implication produces some results that are, well, odd.  These paradoxes were pointed out by C. I. Lewis in 1912 (before Whitehead and Russell had even gotten all three of their volumes published). 
  1. Anything materially-implies a true thing, whether or not the first thing is connected with the second or whether or not the first thing is even true. 
    α→(β→α)

    Examples that are true since strawberries are red: 

    • If the moon is made of green cheese then strawberries are red. 
    • If the sky is blue then strawberries are red. 
  2. A false thing materially-implies anything and everything. 
    ¬α→(α→β)

    Examples that are true since the moon is not made of green cheese: 

    • If the moon is made of green cheese then strawberries are red. 
    • If the moon is made of green cheese then pigs can fly. 

One may argue that each of these examples is obviously true, assuming that material implication is what was meant.  That doesn't seem to be very useful, though.  Why use → to express "if ... then ..." if it doesn't mean what people ordinarily mean by "if ... then ..."?  Reasoning almost always uses some property of the things being reasoned about that is not included in their truth values, and on that basis it is not helpful to try to express that reasoning using →. 

[Epstein2001-pl p. 89 ff.]

parenthesization
It is safest to completely parenthesize, but this it tedious both for writer and reader and in practice parentheses are omitted whenever possible.  Unfortunately there is no single convention for doing so. 

It appears universal that ¬ binds the tightest and always applies to the next symbol, negation, or parenthesized expression;  and that ∧ and ∨ bind tighter than any remaining operators.  However there is no established custom for the relative priority of ∧ and ∨.  It appears safest to use parentheses to disambiguate them.  Here are some conventions, beginning with the most recent: 

  • [Bell+DeVidi+Solomon2001-lo p.8]:  ¬ tightest, then ∧, then ∨, then →. 
  • [Epstein2001-pl p.14]:  ¬ binds tightest, then ∧ and ∨ next (and of equal strength, in order left to right), then any other logical operator. 
  • [Barwise+Etchemendy1992-lfol p. 42]:  parentheses are required in expressions containing both ∧ and ∨. 
  • [Quine1982-ml p.29]:  negation tightest, then conjunction, then all others. 
  • [Kleene1967-ml p.7]:  ¬ tightest, then ∨, then ∧, then ⊃, then ∼. 

See the table of logical symbols

predicate
A predicate takes one or more domain objects and produces either true or false as a result.  Predicates are thus what connects objects and formulae. 
propositional variable
A variable whose value may be true or false and which represents any proposition “whose internal structure won’t be under consideration” [Epstein2001-pl p.13]. 

Also known as a sentence letter

quantification
Quantification is used to state properties of a formula over all objects in a domain.  The existential quantification ∃x F(x) is true if there is some object a in the domain for which F(a) is true, and the universal quantification ∀x Fx is true if F(x) is true for every object in the domain, or more exactly it there is no object a in the domain for which F(a) is false. 

Quantifiers use variables as parameters to direct the ranging over the domain.  A quantifier is said to bind the variable that is its parameter [Barwise+Etchemendy1992-lfol p.115]. 

quantifier
Either an existential quantifier or a universal quantifier.  A quantifier is used to make an assertion about the application of a formula to every object in the domain:  either that the formula is true for every object in the domain (a universal quantification) or that it is true for some object in the domain (an existential quantification).  A quantifier also binds its variable
reference
See designation
satisfiable
A sentence S is satisfiable if there is some interpretation in which S is true [Boolos+Jeffrey1989-cl p.104]. 

Satisfaction can be generalized to sets ΓS1,S2,...,Sn} of sentences: Γ is satisfiable if there is an interpretation I that satisfies every sentence in Γ [Boolos+Jeffrey1989-cl p.105].  (Note that by this definition ∅ is satisfiable.)

Quine uses consistent for the closely related concept of a truth-functional schema that is true for at least one interpretation of its (sentence) letters; he names the three possibilities inconsistent (false for every sentence letter interpretation), consistent (true for some interpretation), and valid (true for all interpretations).  [Quine1982-ml p.40]. 

Compare unsatisfiable, valid

Always true valid satisfiable
Sometimes true,
sometimes false
unsatisfiable
Always false invalid
schema
(Plural schemata or schemas.)

In appearance like a formula, but in which letters stand not for atomic sentences but for subformula whose specifics are abstracted away for now [Quine1982-ml p.33]. 

A schema thus stands for (typically) an infinite number of actual formulae, in which each letter of the schema has been replaced by some subformula. 

Where the letters of the schema are replaced only by “true” or “false”, no complication ensues.  However, where the letters are replaced by formulae, caution must be used as the formulae may not be independent of each other.  For example, an inconsistent schema remains inconsistent (and a valid schema remains valid) if its letters are replaced by formulae; but a consistent schema does not necessarily remain consistent if its letters are replaced by formulae, because the dependence among the subformulae may make some lines of the schema’s truth table impossible [Quine1982-ml p.44]. 

Epstein uses metavariables as components of formulae; a formula containing one or more metavariables appears to be indistinguishable from a schema. 

Epstein also uses “schema” as its own plural, rather than either “schemata” or “schemas.”

second-order logic
A logic in which quantification is extended beyond domain objects to functions, predicates, and/or operations [Bell+DeVidi+Solomon2001-lo p. 122]. 

A well-known example of a second-order statement is Leibniz's definition of identity ∀x ∀y [ x=y ↔ ∀P (Px ↔ Py) ]

See first-order logic

sentence
A sentence is a formula containing no free variables [Boolos+Jeffrey1989-cl p.101, Barwise+Etchemendy1992-lfol p.115]

A sentence has one of the following seven forms, and contains no free variables [Boolos+Jeffrey1989-cl p.101].  In the list of forms, S, S1, and S2 are also sentences;  F is a formulaLS is a sentence letterLP1, LP2, LP3, ... are one-, two-, three-, ... place predicate letters;  and T1, T2, ... are terms

Sentences
1. -S (negation)
2. (S1&S2) (conjunction)
3. (S1S2) (disjunction)
4. S1→ S2) (material implication)
5. (S1↔ S2) (material biconditional)
6. ∀v F (universal)
7. ∃v F (existential)
8. LS (sentence letter)
9. T1=T2 (identity predicate)
10. LP1 T1 or
T1 LP2 T2 or
LP3(T1,T2,T3) or ...
(predicate)

Because a sentence may have no free variables, the formula F in items 6 and 7 must have (at most) one free variable, and it must be the quantifier parameter v

Quine uses statement or closed sentence for this concept, and open sentence for what we are terming a formula [Quine1982-ml p.134]. 

Compare formula

sentence letter
A sentence letter is a constant reference standing for either true or false [Boolos+Jeffrey1989-cl]. 

This is termed an elementary statement or statement letter by Bell et al. [Bell+DeVidi+Solomon2001-lo p.6-7], and a propositional variable by Epstein, who says they stand for any proposition “whose internal structure won’t be under consideration” [Epstein2001-pl p.13]. 

Compare name which is a constant reference to a domain object. 

See interpretation which (among other things) assigns true or false to each sentence letter. 

Skolem-Löwenheim theorem
(Of first-order logic.)  If an enumerable set Δ of sentences has a model, then it has a model with an enumerable domain [Boolos+Jeffrey1989-cl p.131]. 
sound
A proof system or decision process is sound if it classifies as valid only those sentences that are in fact valid. 
statement
See sentence
statement letter
See sentence letter.  [Bell+DeVidi+Solomon2001-lo p.6-7]. 
structure
See interpretation
tautology
A truth-functional schema is a tautology if it is true for every interpretation of its (sentence) letters

Compare valid

term
A term is something that refers to a domain object (see interpretation): either a name, or an n-place function symbol applied to an n-tuple of terms as its parameters [Boolos+Jeffrey1989-cl p.102]. 
theory
A theory T is a set of all sentences in some language K that are implied by T itself [Boolos+Jeffrey1989-cl p.106]. 

A theory always contains all valid sentences of K, as these are implied by any set of sentences of K.  Thus, for example, every theory contains ∀x (x=x) as this is valid in every language.  These sentences can be thought of as the basis of the theory, and that the theory, which by definition is closed under the operation of implication, can be constructed by applying implication successively using the sentences already in the theory. 

Note that in general there are true sentences in K that are not in a particular theory T.  In particular, for every theory of any language that includes the language of arithmetic, there are true statements that are not in the theory.  See complete

truth table
A table presenting a collection of true and false values for the components of a sentence, and the corresponding values of the sentence.  Some examples of simple truth tables appear at the entries for conjunction, disjunction, and negation.  In these tables, the components are simply sentence letters

Truth tables are also used for analyzing more complex sentences by hand.  Then the truth values of the smallest sub-sentences are filled in under the operator that defines them, continuing to larger sub-sentences until the entire table is filled (or until the desired result is obtained, for example demonstrating satisfiability).  An example table appears below: first the possible values for A, B, and C are entered, then the values for (A∧B) and ¬ C; finally, the values for (A∧B)∨ ¬ C are entered under the [Barwise+Etchemendy1992-lfol p.53-4]. 

Empty table
A B C (A∧B) ¬ C
 
 
 
 
 
 
 
 
A, B, and C entered
A B C (A∧B) ¬ C
T T T
T T F
T F T
T F F
F T T
F T F
F F T
F F F
(A∧B), ¬ C entered
A B C (A∧B) ¬ C
T T T T F
T T F T T
T F T F F
T F F F T
F T T F F
F T F F T
F F T F F
F F F F T
(A∧B)∨ ¬ C entered
A B C (A∧B) ¬ C
T T T T T F
T T F T T T
T F T F F F
T F F F T T
F T T F F F
F T F F T T
F F T F F F
F F F F T T
truth value
Either true or false. 

The table of logical symbols lists the symbols used by various authors for true and false. 

truth-functional
Having a truth value that depends solely upon the truth values of its components and the structure in which these components are combined. 

All formulae as defined here are truth-functional (e.g. conjunction, disjunction, material biconditional, etc.), but natural language sentences are generally not (e.g. “Don’t crush that dwarf, hand me the pliers”). 

“Being truth-functional means that the truth value of a complex sentence built up using these connectives depends on nothing more than the truth values of the simpler sentences from which it is built” [Barwise+Etchemendy1992-lfol p.35]. 

universal quantification
A formula constructed from a variable x (the parameter of the quantification) and a subformula F.  The universal quantification is true if and only if the subformula F is true for all possible values of the variable x (all objects in the domain). 

A universal quantification is written as ∀x (F); occasionally this notation is seen ∀x.F, but it is obsolete (see dot). 

The quantifier ∀x is said to bind x (see bound variable, free variable).  Any unbound instance of x in F becomes bound in ∃x (F)

See universal quantification, quantification, existential quantifier

universal quantifier
∀x for some variable x. 
unsatisfiable
A formula is unsatisfiable if it is not satisfiable; that is, if it is true for no interpretation. 

See satisfiable, valid

valid
A sentence is valid if it is true in every interpretation [Boolos+Jeffrey1989-cl p.104]. 

If S is valid, we write ⊢S [Boolos+Jeffrey1989-cl p.104]. 

Compare tautology

See satisfiable, unsatisfiable

variable
A letter that can refer to any object in the domain of an interpretation.  Variables are a part of the logical symbols.  A variable v in a formula F becomes bound if F is a subformula of a quantifier whose parameter is v; otherwise, v is free

See bound variable, free variable; compare metavariable, name, sentence letter

well-formed formula
A well-formed formula, also called a wff (pronounced “wiff”), is a syntactically-correct formula [Epstein2001-pl p.13]. 

The concept of a wff seems to demand that a formula may potentially be ill-formed; other authors appear to use “formula” for “wff,” or for emphasis use “syntactically correct formula.”

Historical notes

A somewhat biased and incomplete summary of the history of logic:

The study of logic dates back about two and a half millennia to Aristotle.  From the perspective of the current day, Aristotleian logic seems to consist of some propositional logic without a good notation and a proof system in which the inference rule is essentially the subset operation.  From this substantial beginning nothing of much interest developed until Frege developed a first-order formal logic that supported quantifiers and n-ary predicates and functions, and in which semantics followed syntax.  For a time logic appeared to be primarily the study of mathematical proof systems, led by Whitehead and Russell and to a lesser extent Hilbert.  Gödel’s Incompleteness Theorem gave this program a severe setback, but the view that logic is the handmaiden to mathematical proof continues to thrive (to some extent, for example, in Bell et al. [Bell+DeVidi+Solomon2001-lo]).  The main thrust of logic, however, shifted to computability and related concepts, models and semantic structures, expressiveness, extensions of classical logic for other situations, and the study of logical systems as subjects of interest in their own right. 

One of the motivations for the present work is the diversity in symbology, terminology, and ontology of concepts among prominent authors.  Thus is is of interest to see where and when various approaches originated, and consider how they have affected the present day.  Most of the information about symbols and approaches from before 1950 are from Quine [Quine1982-ml], who gives a historical note at the end of most sections. 

(It is interesting to note how the points of view of various authors, including me, is expressed in their views of what was important in the history of logic.  One important point of view is “logic as the basis of mathematical proofs”; this point of view seems to go along with the idea that modern logic began with Boole, and that Frege was unimportant.)

Examples

  1. ‘Anyone who loves himself loves someone’  {L:loves}

    ∀x (xLx → ∃y xLy)  [Boolos+Jeffrey1989-cl p.96]. 

  2. ‘None of Alma’s lover's lovers love her’  {a:Alma, L:loves}

    ∀x [ ∃y (xLy & yLa) → −xLa]  [Boolos+Jeffrey1989-cl p.96].  I would say that this is expressed by

    ∀x [ −∃y (xLy & yLa) → xLa]

  3. ‘Nobody has more than one father’  {F:‘is a father of’}

    ∀x ∀y ∀z [ (xFz&yFz) → x=y]  [Boolos+Jeffrey1989-cl p.96]. 

  4. ‘Alma loves her paternal grandfather’  {a:Alma, F:‘is a father of’, L:loves}

    aLf(f(a))  [Boolos+Jeffrey1989-cl p.97]. 

  5. ‘Everyone thinks everyone is an only child’  [Kripke, quoted in Preti2003-k p.5]. 

References

Barwise+Etchemendy1992-lfol
Jon Barwise and John Etchemendy.  The Language of First-Order Logic.  Third edition. CSLI Publications, Stanford, California.  1992. 
Bell+DeVidi+Solomon2001-lo
John L. Bell, David DeVidi, and Graham Solomon.  Logical options.  Broadview, Peterborough, Ontario.  2001. 
Boolos+Jeffrey1989-cl
George S. Boolos and Richard C. Jeffrey.  Computability and Logic.  Third edition. Cambridge University Press.  1989. 
Church1958-iml
Alonzo Church.  Introduction to Mathematical Logic.  Second edition. Princeton University Press.  1958. 
Epstein2001-pl
Richard L. Epstein.  Propositional Logics.  Second edition. Wadsworth, Belmont, CA, USA.  2001. 
Kleene1967-ml
Stephen Cole Kleene.  Mathematical Logic.  Wiley and Sons.  1967. 
Preti2003-k
Consuelo Preti.  On Kripke.  Wadsworth.  2003. 
Quine1982-ml
W. V. Quine.  Methods of Logic.  Fourth edition. Harvard University Press.  1982. 
Tarski1949-ilmd
Alfred Tarski.  Introduction to Logic and to the Methodology of Deductive Sciences.  Second edition. Oxford University Press, New York.  1949. 
Whitehead+Russell1910-13-pm
A. N. Whitehead and B. Russell.  Principia mathematica.  Cambridge University Press.  1910-13. 
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2010May16Su21:42
Thomas A. Alspaugh