anthony morris


  • An attempt to justify a conclusion by rational means
  • Set of statements consisting of:
    • One or more premises (supporting statements)
    • A conclusion (statement in need of support)
  • Two forms of arguments:
  • A bad argument can have a true conclusion
  • "Is this a good argument?" !== "Do you agree with the conclusion?"

Forms of Arguments

Deductive Arguments

  • Intends to provide conclusive support for its conclusion
    • When providing conclusive support for its conclusion, it is valid
    • When not providing conclusive support for its conclusion, it is invalid
    • Validity is all or nothing
  • If premises provided are true, the conclusion must be true
    • Are premises true?
    • Do the premises lead to this conclusion?
  • True premises aren't enough (must be valid)
    • Gasoline is poison (true)
    • Bleach is poison (true)
    • Conclusion: gasoline is bleach (false)
  • Deductively valid arguments with true premises are sound
    • Valid arguments are not necessarily true (sound)!


  • Unjustly killing innocent people is wrong
  • THe holocaust was a case of unjustly killing innocent people
  • Conclusion: Holocaust was wrong

Non-deductive Arguments

  • Tries to provide probable support for its conclusion
    • A conclusion should seem more likely to be true
    • Not definitive truth
  • A non-deductive argument providing probable support is strong
    • Strength comes in degrees
  • A strong argument with true premises is cogent
    • Cogency runs parallel to soundness
    • Cogent arguments can have false conclusions

Inductive Arguments

  • Pattern probably holds in general based on an existing observation

Abductive Arguments

  • A theory is probably true because the theory is the best that exists for the conclusion
  • Compared to the alternatives the argument is best
  • Inference to the best explanation
  • Many philosophers think scientific theories are commonly supposed by abduction
  • Does not positively verify the conclusion
  • Famous scientific examples
  • "When you have eliminated the impossible, whatever remains, however improbable, must be the truth." ~ Sherlock Holmes

Analogical Arguments

  • Attempts to establish that something is probably true of a thing due to similarity to another thing

Ontological Arguments

  • Typically refers to Anselm's original argument for the existence of God
  • Attempts to establish that God exists from reason alone
    • No reliance of empirical observation or experiment
    • Arguments like this can be said to be a priori

Reconstructing Arguments

  • Piece together an argument from a passage of text
  • There may be missing pieces
    • Hidden premises or conclusions
    • Can happen due to various reasons:
      • Lazy author
      • Think premise(s) are obvious
  • Look for a conclusion indicator (eg. so)
  • How to find missing premises
    • Search for credible premise that would make argument valid (or as strong as possible)
    • Choose premise that is:
      • Most plausible
      • Fits best with author's intent
      • Principle of Clarity
        • Always attribute the strongest possible interpretation of an author's position consistent with the text

Evaluating Arguments

  • Think about two questions:
    • Are the premises of this argument true?
    • Assuming the premises are true, to what extend do they support the conclusion?
  • Look out for faulty reasoning
    • A fallacy
    • Error in reasoning that commonly persuades people

Begging the Question

  • Attempt to establish conclusion of an argument by using the conclusion as a premise
  • Usually some disguised version of the conclusion is a premise