Tuesday, October 07, 2008

Spot the fallacy

A blog headline reads "One-fourth of mammals are facing extinction". Now let's say this is true. I was musing as I read this headline whether they had included humans in this assessment. So I have a syllogism for you:
One-fourth of mammals face extinction
Humans are mammals
Therefore one-fourth of humans face extinction
I'm thinking, given their current polls, that Republicans may find themselves among the unfortunate quartile.

Okay, okay, I know that that the headline should have indicated is that one-fourth of animal species are facing extinction, but I'm still interested to now if anyone can spot what is wrong with this argument.


Anonymous said...


Anonymous said...

Well, prof, even assuming that the first statement is true, humans do not make up any where near one quarter of the mammal population, so the final statement is false.

Martin Cothran said...

That, of course, is why we know we have a fallacy on our hands: because the premises are true and the conclusion false. But the question is still what is wrong with the reasoning.

Kari said...

The problem is with the second statement. If it was reversed and read "Mammals are Humans" then it would be true.

Martin Cothran said...


No. All that would do is give you a false premise in addition to a false conclusion, and would make it formally invalid since it would (in addition to the problem it already has) make it commit the fallacy of illicit minor.

I'll explain it tomorrow if no one else does.

Anonymous said...

"Some mammals are facing extinction
All humans are mammals
Therefore, some humans are facing extinction"

IAI in sub-prae. Not a valid form. We cannot necessarily deduce that humans are part of the group of mammals that are facing extinction.


Niggardly Phil said...

undistributed middle