Contents

1. Defeasible Arguments
2. Defeaters
3. Two Kinds of Defeaters
4. Deductive Arguments Are Not Defeasible

Defeasible Arguments

• The world is complex and reasoning can get complicated. Anything that simplifies matters facilitates reasoning.  One way of simplifying things is to keep other things equal.
• A defeasible argument is an argument whose premises (purportedly) support its conclusion other things being equal (and which support can therefore be “defeated” by additional information).

• Consider the arguments:
  • An Evidential Argument:
    • In 1998 CNN reported that the U.S. had used sarin nerve gas in Laos in 1970 as part of Operation Tailwind during the Vietnam War.
    • CNN is a reliable source of news.
    • Therefore, the US used sarin gas in Laos in 1970.
  • A Normative Argument:
    • You promised you would take your little brother to the zoo this morning.
    • Promises should be kept.
    • Therefore you should take your little brother to the zoo this morning.
  • An Analogical Argument:
    • Three recently purchased houses in the neighborhood sold for an average 250K.
    • Your house is like those houses in terms of square footage, number of bedrooms, and age.
    • Therefore your house will likely sell for about 250K

• Though the premises support their conclusions in different ways, the arguments have in common that the support of the conclusion by the premises can be nullified or rebutted by additional information.
• Thus the support of the conclusions is “defeated” by the following facts:
  • For the CNN argument:
    • the fact that CNN retracted the story;
      • cnn.com/US/9807/02/tailwind.johnson/
  • For the argument about taking your brother to the zoo:
    • the fact that you are in the hospital this morning;
  • For the argument about the likely selling price of your home:
    • the fact that there’s black mold throughout your house.

Defeaters

• A proposition D defeats an argument if
  • the premises of the argument support the conclusion
  • but the premises of the argument with additional premise D don’t support the conclusion.
• Consider the CNN argument again:
  • In 1998 CNN reported that the U.S. had used sarin nerve gas in Laos in 1970 as part of Operation Tailwind during the Vietnam War.
  • CNN is a reliable source of news.
  • Therefore, the US used sarin gas in Laos in 1970.
• The premises support the conclusion.
• But CNN’s later retraction of the story defeats that support. Adding the defeater to the original premises results in an argument whose premises don’t support its conclusion:
  • In 1998 CNN reported that the U.S. had used sarin nerve gas in Laos in 1970 as part of Operation Tailwind during the Vietnam War.
  • CNN is a reliable source of news.
  • CNN retracted its 1998 story about the use of sarin gas.
  • Therefore, the US used sarin gas in Laos in 1970.

Two Kinds of Defeaters

• There are two kinds of defeaters.
• A rebutting defeater defeats an argument by casting doubt on its conclusion.
• An undercutting defeater defeats an argument by nullifying the premises’ support of the conclusion.
• Example:
  • Suppose I have a reliable watch. My watch reads 3:00 and I infer that it’s 3:00.
  • The argument underlying my inference:
    • My watch says it’s 3:00.
    • My watch is reliable.
    • Therefore it’s 3:00.
  • A rebutting defeater is that other people’s watches say 4:00, which casts doubts on it’s being 3:00.
  • An undercutting defeater is that my watch has stopped, which undermines my inference that it’s 3:00 (but isn’t a reason for thinking that it’s not 3:00).

Deductive Arguments Are Not Defeasible

• Deductive arguments are not defeasible. That’s because a valid deductive argument remains valid no matter what or how many premises are added.
• Consider the valid deductive argument:
  • Only natural-born U.S. citizens are eligible to be president.
  • John Oliver isn’t a natural-born U.S. citizen.
  • Therefore, he’s not eligible to be president.
• Let D be any statement. Consider the argument with additional premise D:
  • Only natural-born U.S. citizens are eligible to be president.
  • John Oliver isn’t a natural-born U.S. citizen.
  • D is true.
  • Therefore, he’s not eligible to be president.
• D doesn’t defeat the updated argument because the conclusion follows necessarily from its three premises. And that’s because the conclusion follows necessarily from the first two premises. Deductive arguments cannot therefore be defeated.