# Deductive reasoning

Deductive reasoning, also deductive logic or logical deduction or, informally, "top-down" logic, is the process of reasoning from one or more general statements (premises) to reach a logically certain conclusion.

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 from general statements, but in inductive reasoning the conclusion is reached from specific examples. (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.)

## Simple Example

An example of a deductive argument:

1. All men are mortal.
2. Aristotle is a man.
3. Therefore, Aristotle is mortal.

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

## Law of Detachment

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:

1. P→Q (conditional statement)
2. P (hypothesis stated)
3. 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 valid conclusion.

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

1. If an angle A>90°, then A is an obtuse angle.
2. A=120°
3. A is an obtuse angle.

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

## 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, with the true premise P:

1. P→Q
2. Q→R
3. Therefore, P→R.

The following is an example:

1. If Larry is sick, then he will be absent from school.
2. If Larry is absent, then he will miss his classwork.
3. 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

## Deductive Logic: 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. Trick arguments are based off of this.

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

1. Everyone who eats carrots is a quarterback.
2. John eats carrots.
3. 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." Everyone who eats carrots is NOT 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).

## Hume's Skepticism

Philosopher David Hume presented grounds to doubt deduction by questioning induction. Hume's problem of induction starts by suggesting that the use of even the simplest forms of induction simply cannot be justified by inductive reasoning itself. Moreover, induction cannot be justified by deduction either. Therefore, induction cannot be justified rationally. Consequently, if induction is not yet justified, then deduction seems to be left to rationally justify itself – an objectionable conclusion to Hume.

## Deductive reasoning and 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.