Deductive reasoning - PowerPoint Presentation

Slide1

Deductive reasoningSlide2

Deductive reasoning

, also

deductive logic

or

logical deduction

or, informally,

"

top-down

" logic

, is the process of

reasoning

from one or more

statements

(premises) to reach a logically certain conclusion. It differs from

inductive reasoning

or

abductive

reasoning

.

Deductive reasoning links

premises

with

conclusions

. If all premises are true, the terms are

clear

, and the rules of deductive

logic

are followed, then the conclusion reached is

necessarily true

.

Deductive reasoning (top-down logic) contrasts with

inductive reasoning

(bottom-up logic) in the following way: In deductive reasoning, a conclusion is reached

reductively

by applying general rules that hold over the entirety of a

closed domain of discourse

, narrowing the range under consideration until only the conclusion is left. In inductive reasoning, the conclusion is reached by generalizing or extrapolating from initial information. As a result, induction can be used even in an

open domain

, one where there is

epistemic uncertainty

. Note, however, that the inductive reasoning mentioned here is not the same as

induction

used in mathematical proofs –

mathematical induction

is actually a form of deductive reasoning.Slide3

Simple example

An example of a deductive argument:

All men are mortal.

Socrates is a man.

Therefore, Socrates is mortal.

The first premise states that all objects classified as "men" have the attribute "mortal". The second premise states that "Socrates" is classified as a "man" – a member of the set "men". The conclusion then states that "Socrates" must be "mortal" because he inherits this attribute from his classification as a "man".Slide4

Law of detachment

Modus ponens

The law of detachment (also known as

affirming the antecedent

and

Modus ponens

) is the first form of deductive reasoning. A single

conditional statement

is made, and a hypothesis (P) is stated. The conclusion (Q) is then deduced from the statement and the hypothesis. The most basic form is listed below:

P → Q (conditional statement)

P (hypothesis stated)

Q (conclusion deduced)

In deductive reasoning, we can conclude Q from P by using the law of detachment. However, if the conclusion (Q) is given instead of the hypothesis (P) then there is no definitive conclusion.

The following is an example of an argument using the law of detachment in the form of an if-then statement:

If an angle satisfies 90° < A < 180°, then A is an obtuse angle.

A = 120°.

A is an obtuse angle.

Since the measurement of angle A is greater than 90° and less than 180°, we can deduce that A is an obtuse angle.Slide5

Law of syllogism

The law of

syllogism

takes two conditional statements and forms a conclusion by combining the hypothesis of one statement with the conclusion of another. Here is the general form:

P → Q

Q → R

Therefore, P → R.

The following is an example:

If Larry is sick, then he will be absent.

If Larry is absent, then he will miss his classwork.

Therefore, if Larry is sick, then he will miss his classwork.

We deduced the final statement by combining the hypothesis of the first statement with the conclusion of the second statement. We also allow that this could be a false statement. This is an example of the Transitive Property in mathematics. The Transitive Property is sometimes phrased in this form:

A = B.

B = C.

Therefore A = C.Slide6

Law of contrapositive

Modus

tollens

The law of

contrapositive

states that, in a conditional, if the

conclusion

is false, then the

hypothesis

must be false also. The general form is the following:

P → Q.

~Q.

Therefore we can conclude ~P.

The following are examples:

If it is raining, then there are clouds in the sky.

There are no clouds in the sky.

Thus, it is not raining.Slide7

Validity and soundness

Deductive arguments are evaluated in terms of their

validity

and

soundness

.

An argument is valid if it is impossible for its

premises

to be true while its conclusion is false. In other words, the conclusion must be true if the premises are true. An argument can be valid even though the premises are false.

An argument is sound if it is valid and the premises are true.

It is possible to have a deductive argument that is logically valid but is not sound. Fallacious arguments often take that form.

The following is an example of an argument that is valid, but not sound:

Everyone who eats carrots is a quarterback.

John eats carrots.

Therefore, John is a quarterback.

The example's first premise is false – there are people who eat carrots and are not quarterbacks – but the conclusion must be true, so long as the premises are true (i.e. it is impossible for the premises to be true and the conclusion false). Therefore the argument is valid, but not sound. Generalizations are often used to make invalid arguments, such as "everyone who eats carrots is a quarterback." Not everyone who eats carrots is a quarterback, thus proving the flaw of such arguments.

In this example, the first statement uses

categorical reasoning

, saying that all carrot-eaters are definitely quarterbacks. This theory of deductive reasoning – also known as

term logic

 – was developed by

Aristotle

, but was superseded by

propositional (sentential) logic

and

predicate logic

.

Deductive reasoning can be contrasted with

inductive reasoning

, in regards to validity and soundness. In cases of inductive reasoning, even though the premises are true and the argument is "valid", it is possible for the conclusion to be false (determined to be false with a counterexample or other means).Slide8

Education

Deductive reasoning is generally thought of as a skill that develops without any formal teaching or training. As a result of this belief, deductive reasoning skills are not taught in secondary schools, where students are expected to use reasoning more often and at a higher level. It is in high school, for example, that students have an abrupt introduction to

mathematical proofs

 – which rely heavily on deductive reasoning.