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  • Ways of Knowing: Reason


  • Cogito ergo sum

  • ReasoningDeductiveInductive

  • Deductive Reasoning: Syllogism

    All men are mortal Socrates is a manTherefore Socrates is mortal

  • All blims are blamsSome blims are blomsTherefore some blams are bloms

  • - if the premises are valid- and the argument is valid- then the conclusion is valid

  • All panthers are pinkChe Guevara is a pantherTherefore Che Guevara is pink

  • Inductive reasoning

  • Deduction and induction compared




    Reasoning from general to particular

    Reasoning from particular to general


    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.


    More certain, but less informative than induction

    More informative, but less certain than deduction.

  • The Bases of Reason: The Laws of Thought1 The law of identity: Whatever is, is2 The law of contradiction: Nothing can both be and not be3 The law of excluded middle: Everything must either be or not be

  • a priori knowledge

  • The Value of Reasonrationalism- 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 liarsI am a CretanTherefore 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