Thursday, 17 April 2014

Critical Thinking 10: Affirming the Consequent

Higher Philosophy Podcast
Critical Thinking: Affirming the Consequent

This is available as an MP3 or to download as a podcast through iTunes
for Desktop and Apple devices, as well as PodcastHD for Android Devices.

We are more familiar with this fallacy than we know. Consider the following:

If someone is a turtle then they are a ninja, Donatello is a ninja, so Donatello is a turtle.

Reconstruction of the argument reveals its invalidity:

P1      If someone is a turtle (P), then they are ninja (Q).
P2      Donatello is ninja (Q).
C        Donatello is a turtle (P).

We know from looking at the argument it is invalid. All turtles may be ninjas, but not all ninjas are turtles.  As we refer to Q as the consequent, and since P2 affirms this, the fallacies title is affirming the consequent. Therefore the structure P→Q,Q∴P is not only invalid but a formal fallacy.

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