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
