9. Formal and Informal Fallacies

 

Wireless Philosophy, Fallacies, 7:05. Original title: CRITICAL THINKING - Fallacies: Formal and Informal Fallacies

Summary

Paul Henne (Duke University) opens the fallacies part of the course with two arguments that both feel wrong, but for entirely different reasons:

Premise 1: If someone is allergic to peanuts, then she doesn't eat peanut butter.
Premise 2: Jane doesn't eat peanut butter.
Conclusion: Therefore, Jane is allergic to peanuts.

Premise 1: A feather is light.
Premise 2: What's light cannot be dark.
Conclusion: Therefore, a feather cannot be dark.

A fallacy is a defect in reasoning. It can be unintentional, a simple mistake, or intentional, an attempt to trick someone into a belief through deceptive reasoning. Either way, being able to identify the distinct types is what lets us correct the defects in our own thinking.

A reminder: valid forms

Premise 1: If someone is a philosopher, then she publishes articles in philosophy.
Premise 2: Tamar Gendler is a philosopher.
Conclusion: Therefore, Tamar Gendler publishes articles in philosophy.

This is valid, and its form is called modus ponens: if X then Y; X; therefore Y. As always, the valid form does not make the premises true; the argument would have been just as valid had she become a train conductor instead of a philosopher.

Formal fallacies

A formal fallacy is exactly what it sounds like: a defect in the form of the argument. The peanut butter argument commits one called affirming the consequent:

If X then Y.
Y.
Therefore X.

The defect becomes obvious once you notice that Jane might simply dislike peanut butter. Given only that she doesn't eat it, plus the conditional in the first premise, her allergy does not follow. (This is one of the fallacies frequently tested on the LSAT.)

The mark of a formal fallacy is that content is irrelevant to it. Anything plugged into a bad form yields an invalid argument, right down to "if meow, then splat; splat; therefore meow".

Informal fallacies

The feather argument is different: the defect arises from the content, not the form. At first glance the form looks fine, and a genuinely similar-looking argument is valid:

Premise 1: Rain is wet.
Premise 2: What's wet cannot be dry.
Conclusion: Therefore, rain cannot be dry.

The trouble with the feather is the two meanings of "light", light in weight and light in colour. Ignoring the meaning, the form appears to be all X are Y; what is Y cannot be Z; therefore no X are Z. Taking the shift in meaning into account, the real form is all X are Y; what is W cannot be Z; therefore no X are Z, which is invalid. This fallacy is equivocation, and the only way to detect it is by examining the content: the form alone will never reveal it.

Key takeaway

A formal fallacy is a defect in the argument's form, so any argument of that shape is invalid. An informal fallacy is a defect in the argument's content, which may in turn produce a defect in its form. An argument can therefore have true premises and an apparently valid form and still commit a fallacy.