# logic

Logic is the branch of philosophy and mathematics concerned with analyzing the rules that govern correct and incorrect reasoning or inference.

## Historical development of logic

The first major exponent of logic was Aristotle,
who analyzed terms and propositions and in his *Prior Analytics* set out systematically the various forms of syllogism;
this work has remained an important part of logic ever since. Aristotle's
other great achievement was the use of symbols to expose the form of an
argument independently of its content. Thus a typical Aristotelian syllogism
might be: all *A* is *B*; all *B* is *C*; therefore
all *A* is *C*. This formalization of arguments is fundamental
to all logic.

Aristotle's pupil Theophrastus developed
syllogistic logic, and soma of the Stoics used symbols to represent not
single terms but whole propositions, but apart from this there were no significant
developments in later antiquity or the early Middle Ages, although logic
(dialectic) was part of the *trivium*. From the 12th century onward
there was greater revival of interest in logic: Latin translations of Aristotle's
logical works (collectively called the *Organun*) were studied intently,
and a kind of program emerged, which was based on Aristotle and included
much that would nowadays be regarded as grammar, epistemology,
and linguistic analysis. This Scholastic period was a great age of commentaries
and compendiums, with much refinement and minute analysis but little original
work. Among the most important medieval logicians were William
of Ockham, Albert of Saxony, and Jean Buridan.

After the Renaissance an anti-Aristotelian reaction set in, and logic was given a new turn by Petrus Ramus and by Francis Bacon's prescription that induction (and not deduction) should be the method of the new science. In the work of George Boole and Gottlob Frege the nineteenth century saw a vast extension in the scope and power of logic. In particular logic became as bound up with mathematics as it was with philosophy. Logicians became interested in whether particular logical systems were either consistent or complete. (A consistent logic is one in which contradictory statements cannot be validly derived.) The climax of 20th-century logic came in the early 1930s when Kurt Gödel demonstrated both the completeness of Frege's first-order logic and that no higher-order logic could be both consistent and complete.

## Some important terms and topics in logic

*a priori*

The terms *a priori* and *a posteriori* are descriptive of
knowledge or reasoning, reflecting whether or not it is the result of our
experience of the real world. Alleged knowledge attained solely through
reasoning from arbitrary principles is *a priori* (Latin: from earlier
things); that gained empirically, from observation or experience, is *a
posteriori* (Latin: from later things). This modern usage of the terms
derives from the philosopher Kant.

### argument

A mathematical proof, possibly an informal one.

### axiom

A statement or proposition that is considered to be true without need of proof. An axiom need not be self-evident but
should be consistent with the other axioms of a logical system. The term "axiom" comes from the Greek *axios* meaning "worthy" and
was used by many Greek philosophers and mathematicians, including Aristotle.
Curiously, Euclid, whose axioms are best
known of all, seems to have favored a more general phrase meaning "common
notion."

Closely related is the **postulate** which is a less arbitrary
or basic assumption, provisionally accepted for some particular purpose
but more freely open to substitution.

### axiomatic method

A method of mathematical reasoning based on logical deduction from assumptions known as axioms. The method is fundamental to the philosophy of mathematics. It was used by the Greeks and formalized in the early twentieth century by David Hilbert. In an axiomatic system, certain undefined entities (terms) are taken and described by a set of axioms. Other, often surprising, relationships (theorems) are then deduced by logical reasoning.

### complete

Describes a formal system in which all statements can be proved as being true or false. Most interesting formal systems are not complete, as demonstrated by Gödel's incompleteness theorem.

### conjecture

A mathematical statement that has been put forward as a true statement, but that no one has been able yet to prove or disprove; in mathematics, a conjecture and a hypothesis are essentially the same thing. When a conjecture has been proven to be true, it becomes known as a theorem. Famous conjectures include the Riemann hypothesis, the Poincaré conjecture, the Goldbach conjecture, and the twin primes conjecture. Just to show how terminology can be used inconsistently, however, the most famous of all conjectures, for centuries before its proof in 1995, was always known as Fermat's last theorem!

### conjunction

The assertion that two
statements are both true. For statements *a* and *b*,
this is written *A* Λ *B*, read "*a* is
true and *b* is true."

### consistency

An axiomatic theory is said to be consistent if it's impossible (within the confines of the theory) to prove simultaneously a statement and its negation. Gödel's incompleteness theorem states that any (sufficiently powerful) consistent axiomatic theory is incomplete.

### contingency

The property whereby a statement may be either false or true, its truth or falsity depending on the actual state of the world. The statement "the paper in this book is white" is contingent in that although it is in fact true, it is conceivable that it might have been, say, pale green. Compare with necessity.

### disjunction

A logical proposition produced by joining two simple propositions by the
word "or". An example is the proposition "John is intelligent or John is
modest"; it is false if both parts are separately false, otherwise it is
true. The disjunction is used in mathematics – in which the proposition
is true if *either* or *both* components are true. The disjunction
of two simple propositions, *P* or *Q*, is written *P* ^ *Q*, and read "P or Q".

### Euler diagram

A simple diagram used in logic to illustrate syllogisms. Classes of objects are represented
by circles so that, for example, a premise of the type "some *a* is *b*" can be represented by an overlap of two circles, one which
represents *a* and the other representing *b*. See also Venn
diagram.

### excluded middle law

A law in (two-valued) logic which states that
there is no third alternative to truth or falsehood. In other words, for
any statement *A*, either *A* or not-*A* must be true
and the other must be false. This law no longer holds in three-valued logic,
in which 'undecided' is a valid state, nor does it hold in fuzzy
logic.

### existence

A term that has several different meanings within mathematics. In the broadest sense there is the question of what it means for certain concepts, such as pi, to exist. Was π, for example, invented or discovered? In other words, does π exist only as an intellectual construct or was it somehow already "out there" waiting for people to find it. If it does exist independently of the human mind, when did its existence start? Does π predate the physical universe?

Such ontological questions become even more difficult when applied to more
complex or abstract mathematical concepts such as the Mandelbrot
set, surreal numbers, or infinity.
A narrower and more technical type of "existence" in math is implied by
an *existence theorem*. Such a theorem is used to prove that a number
or other object with particular properties definitely exists, but does not
necessarily give a specific example. Finally, there is existence in the
sense of particular solutions to problems. If at least one solution can
be determined for a given problem, a solution to that problem is said to
exist. Something of the flavor of all three types of mathematical existence
mentioned here are captured in the following anecdote:

An engineer, a chemist, and a mathematician are staying in three adjoining cabins at an old motel. First the engineer's coffee-maker catches fire. He smells the smoke, wakes up, unplugs the coffee maker, throws it out the window, and goes back to sleep. Later that night the chemist smells smoke too. He wakes up and sees that a cigarette butt has set the trash can on fire. He thinks to himself, "How does one put out a fire? One can reduce the temperature of the fuel below the flash point, isolate the burning material from oxygen, or both. This could be accomplished by applying water." So he picks up the trash can, puts it in the shower stall, turns on the water, and, when the fire is out, goes back to sleep. The mathematician has been watching all this out the window. So later, when he finds that his pipe ashes have set the bed sheet on fire, he's not in the least taken aback. "Aha!" he says, "A solution exists!" and goes back to sleep.

### formal system

A mathematical formalism in which statements can be constructed and manipulated with logical rules. Some formal systems, such as Euclidean geometry, are built around a few basic axioms and can be expanded with theorems that can be deduced through proofs.

### fuzzy logic

A departure from classical two-valued logic in which something is either true or false, to allow a continuous range of truth values. Fuzzy logic was introduced by Lotfi Zadeh of the University of California at Berkeley in the 1960s as a means to model the uncertainty of natural language.

### Gödel's incompleteness theorem

In a nutshell: All consistent axiomatic systems contain undecidable propositions. What does this mean? An axiomatic system consists of some undefined terms, a number of axioms referring to those terms and partially describing their properties, and a rule or rules for deriving new propositions from already existing propositions.

There are a couple of main reasons why axiomatic systems are so useful:
first, they're compact descriptions of the whole field of propositions derivable
from the axioms, so large bodies of math can be compressed down into a very
small compass; second, because they're so abstract, these systems let us
derive all, and only, the results that follow from things having the formal
properties specified by the axioms. An axiomatic system is said to be *consistent* if, given the axioms and the derivation rules, it doesn't lead to any contradictory
propositions. One of the first modern axiomatic systems was a formalization
of simple arithmetic (adding and multiplying whole numbers), achieved the
great logician Giuseppe Peano and now known
as **Peano arithmetic**. What Kurt Gödel did was to show that every syntactically correct proposition in Peano arithmetic
can be represented by a unique integer, called its **Gödel number**.
(The trick is to replace each symbol in the proposition, *including numerals*,
by a different string of digits. If we represent "1" by 01, "2" by 02, "+"
by 10, and "=" by 11, then the Gödel number of "1 + 1 = 2" is 0110011102.
Gödel numbers tend to be huge!) This lets us write down, unambiguously,
propositions that are about propositions. In particular, we can write down
self-referential propositions – ones that include their own Gödel
number. From here, Gödel showed that, *either* the system is inconsistent, *or* there are true propositions that can't be reached from the axioms
by applying the derivation rules. The system is thus **incomplete**,
and the truth of those propositions is **undecidable** (within
that system). Such undecidable propositions are known as **Gödel
propositions** or **Gödel sentences**. Nobody knows
what the Gödel sentences for Peano arithmetic are, though people have
their suspicions about Goldbach's conjecture (every even number is the sum of two prime numbers).

So far we've just been talking about Peano arithmetic, but results about an axiomatic system apply to all kinds of things that satisfy the axioms. There are an immense number of other axiomatic systems, which either include Peano numbers among their basic entities or can be constructed from them. (These systems are said to provide models of Peano arithmetic.) It follows that these systems, too, contain undecidable propositions, and are incomplete.

### implication

A logical proposition of the type "if *P* then *Q*", connecting
two simple propositions *P* (the **antecedent**) and *Q* (the **consequent**). In the form used in mathematical logic the two propositions don't need to be
connected. This is called **material implication** –
an example is "if the Earth is flat then gold is a metal". A material implication
is false only when the antecedent is true and the consequent false, otherwise
it is true.

In normal discourse **formal implication** is used, in which
the simple propositions are related in meaning; for example, "if it does
not rain, we will go for a walk."

An implication is written as *P* → *Q*, read "*P* implies *Q*."

### induction

A method of reasoning by which one infers a generalization from a series of instances. Say there is a hypothesis H that contains the variable n, which is a whole number. To prove by induction that H is true for every value of n is a two-step process: (1) prove that H is true for n = 1; (2) prove that H being true for n = k implies that H is true for n = k + 1. This is sufficient because (1) and (2) together imply that H is true for n = 2, which, from (2), then implies H is true for n = 3, which implies H is true for n = 4, and so on. H is called an inductive hypothesis. Some philosophers don't accept this kind of proof, because it may take infinitely many steps to prove something; however, most mathematicians are happy to use it.

### necessity

As contrasted with contingency, in logic, the property whereby a statement must be either true or false, this depending on the correct use of language in framing the statement. The statement "this leaf of paper has two sides" is a necessary truth since if it had either more or less than two sides it would not be a leaf according to the normal usage of the term.

### necessity and sufficiency

In mathematical analysis, conditions which
describe the validity of a statement. For example, in the case of 4 < *a* < 10 it is *necessary* that, say, 3 < *a* < 11 since if this
were not true then the original statement would not be true; and *sufficient* that 5 < *a* < 9, since is this is true it implies the truth of the
original statement. More generally, in logic,
a condition is necessary for the truth of a statement if the falsehood of
the condition implies the falsehood of the statement, and sufficient if
the truth of the condition implies the truth of the statement.

### proof

A sequence of statements in which each subsequent statement is derivable from one of the previous statements or from an axiom of a formal system. The final statement of a proof is usually the theorem that one has set out to prove.

### quantifier

In symbolic logic, the **universal quantifier** ∀ indicates
"for every" or "for all." For example, ∀ *x A*, *p(x)* means for all *x* belonging to *A*, the proposition *p
* is valid. The existential quantifier ∃ indicates "there exists." So,
for instance, ∃

*x A*,

*p(x)*means there exists at least one

*x*, belonging in

*A*, for which the proposition

*p(x)*is valid.

### quine

Before we put the motion 'that the motion be now
put,' should we not first put the motion "that the motion 'that the
motion be now put' be now put?" (Chairman of the Meeting of the Society
of Logicians)

—anonymous

A term named by Douglas Hofstadter after the Harvard logician Willard van Orman Quine. It can be used either as a noun or a verb.

Quine (noun). A computer program that produces an exact copy of itself (or, alternatively, that prints its own listing.) This means that when the program is run, it must duplicate (or print out) precisely those instructions that the programmer wrote as part of the program, including the instructions that do the copying (or printing) and the data used in the copying (or printing.) A respectable quine – one that doesn't cheat – is not allowed to do anything as underhand or trivial as seeking the source file on the disk, opening it, and copying (or printing) its contents. Although writing a quine is not always easy, and in fact may seem impossible, it can always be done in any programming language that is Turing complete, which includes every programming language actually in use.

Quine (verb). To write a sentence fragment a first time, and then to write it a second time, but with quotation marks around it. For example, if we quine "say", we get "say 'say'"). Thus, if we quine "quine", we get "quine 'quine'", so that the sentence "quine 'quine'" is a quine. In this linguistic analogy, the verb "to quine", plays the role of the code, and "quine" in quotation marks plays the role of the data.

*reductio ad absurdum*

"Reduction to the absurd;" the process of demonstrating that an idea is probably false by first assuming its truth, and then showing how that truth leads to conclusions which can't possibly be true. In A Mathematician's Apology (1941), G. H. Hardy said: "Reductio ad absurdum, which Euclid loved so much, is one of a mathematician's finest weapons. It is a far finer gambit than any chess play: a chess player may offer the sacrifice of a pawn or even a piece, but a mathematician offers the game."

### strange loop

A phenomenon in which, whenever movement is made upward or downward through
the levels of some hierarchial system, the system unexpectedly arrives back
where it started. Douglas Hofstadter has used the strange loop as a paradigm in which to interpret paradoxes
in logic, such as Grelling's paradox and Russell's paradox, and has
called a system in which a strange loop appears a **tangled hierarchy**.

### syllogism

An argument composed of three parts – a major premise, a minor premise, and a conclusion. For example: All men are mortal (major premise). Socrates is a man (minor premise). Therefore, Socrates is mortal (conclusion). The syllogism forms the basis of Aristotle's system of logic, which went unchallenged for over two thousand years. Aristotle believed that by setting out any argument in syllogistic form, it should be possible to avoid fallacies. However, Bertrand Russell discovered several formal errors in the doctrine of syllogism.

### theorem

A major mathematical proposition that has been proved correct. More precisely, a statement in a formal system for which there exists a proof; in other words, a statement implied by a set of postulates, or axioms. On the its dependence on the axioms is proved, a newly-discovered theorem becomes a potential stepping-stone to the discovery and proof of other, similarly dependent, theorems.

### truth table

In logic, a convenient way of displaying the
"truth values" (truth and falsity) of a compound statement as determined
by the truth values of its simple component statements. For example, if
the simple statements *p* and *q* are both true, then the
compound statement *p and q* is true (1); but if either *p* or *q* or both *p* and *q* are false, then *p and
q* is false (0). This is usually displayed in tabular form as shown.

### universe of discourse

The part of the world under discussion; more precisely, the set of all objects presumed or hypothesized to exist for some specific purpose. Objects may be concrete (e.g., a specific carbon atom, Confucius, the Sun) or abstract (e.g., the number 2, the set of all integers, the concept of justice). Objects may be primitive or composite (e.g., a circuit that consists of many subcircuits). Objects may even be fictional (e.g., a unicorn, Sherlock Holmes). The universe of discourse is a familiar concept in logic, linguistics, and mathematics.