Ways of Knowing: Reason

Reason

Cogito ergo sum

Reasoning

• Deductive

• Inductive

Deductive Reasoning: Syllogism

All men are mortal

Socrates is a man

Therefore Socrates is mortal

All blims are blams

Some blims are bloms

Therefore some blams are bloms

- if the premises are valid

- and the argument is valid

- then the conclusion is valid

All panthers are pink

Che Guevara is a panther

Therefore Che Guevara is pink

Inductive reasoning

Deduction and induction compared

Deduction Induction Definition

Reasoning from general to particular Reasoning from particular to general Example

All metal expand when heated. A is a metal Therefore A expands when heated

Metal A expands when heated; metal B expands when heated; metal C expands when heated. Therefore all metals expand when heated.

Value More certain, but less informative than induction More informative, but less certain than deduction.

The Bases of Reason: The Laws of Thought

1 The law of identity: ‘Whatever is, is’

2 The law of contradiction: ‘Nothing can both be and not be’

3 The law of excluded middle: ‘Everything must either be or not be’

a priori knowledge

The Value of Reason

rationalism- Philos. The doctrine or theory that emphasizes the role of reason in knowledge, or claims that reason rather than sense experience is the foundation of certainty in knowledge. Freq. contrasted with empiricism. (OED)

The Limits of Reason: The Cretan Paradox

All Cretans are liars

I am a Cretan

Therefore I am a liar

Can reason be doubted?

‘It is an act of faith to assert that our thoughts have any relation to reality at all’

- G K Chesterton

Can reason be doubted?

‘The law of excluded middle is true when precise symbols are employed, but it is not true when symbols are vague, as, in fact all symbols are’

- Bertrand Russell

Can reason be doubted?

‘You can never step into the same river twice’ - Heraclitus

Logical Fallacies

Phi