Affirming the Antecedent/Consequent

What do those fancy words, ‘antecedent’ and ‘consequent’, mean? Let’s take the argument, ‘If X is true, then Y is true’. The first part of the argument (“if X is true”) is the antecedent, while the last part (“then Y is true”) is the consequent. Quite simple.

Affirming the Antecedent

Affirming the antecedent happens when someone proves (or affirms) the antecedent part of the argument. It is a perfectly valid form of argumentation. Consider the following: “If I ate a whole pizza, I would be full. I just ate a whole pizza. Therefore, I am full.” This is a valid argument since the antecedent (“If I just ate a whole pizza”) is affirmed as true in the second statement (“I just ate a whole pizza”). Thus, the consequent (“I am full”) must follow as true.

Now it may not be true that you would be full after eating a whole pizza, but the argument itself is valid.

affirming-antecedent-consequentAffirming the Consequent

Affirming the consequent, however, is not a valid form of argumentation. Let us take the above example and modify it into one that affirms the consequent: “If I ate a whole pizza, I am full. I am full. Therefore, I just ate a whole pizza.” This is invalid since proving the consequent (“I am full”) does not mean the antecedent (“I just ate a whole pizza”) is therefore true. You could be full for any number of reasons other than eating pizza.

Unfortunately, this fallacy is probably the one most often used by evolutionists. For instance, they might say that “If evolution were true, there would be DNA similarities. There are DNA similarities. Therefore, evolution is true.” Just because there are DNA similarities, that does not mean evolution is true! In order to change this to a valid argument, we would have to say “If evolution were true, there would be DNA similarities. Evolution is true. Therefore, there are DNA similarities.” But of course, this could not be used as evidence for evolution because it assumes evolution is true to begin with!

It is recommended that you memorize the differences between affirming the antecedent (valid) and affirming the consequent (invalid). If this is done, you will be able to deconstruct illogical arguments very easily.

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