What is Affirming the Consequent?

To those who confuse hopelessly the order of horses and carts, affirming the consequent is a fallacy which comes naturally. An occupational hazard of those who engage in conditional arguments, this particular fallacy fails to recognize that there is more than one way of killing a cat.


When cats are bitten by rabid hedgehogs they die. Here is a dead cat, so obviously there is a rabid hedgehog about.

(Before locking up your cats, reflect that the deceased feline might have been electrocuted, garrotted, disembowelled, or run over. It is possible that a rabid hedgehog got him, but we cannot deduce it as a fact.)


The arguer has mixed up the antecedents and consequents. In an ‘if … then’ construction, the ‘if’ part is the antecedent, and the ‘then’ part is the consequent. It is all right to affirm the antecedent in order to prove the consequent, but not vice versa.


If I drop an egg; it breaks. I dropped the egg, so it broke.

(This is perfectly valid. It is an argument called the modus ponens which we probably use every day of our lives. Compare it with the following version.)

If I drop an egg, it breaks. This egg is broken, so I must have dropped it.

(This is the fallacy of affirming the consequent. There could be many other incidents leading to a broken egg, including something falling upon it, someone else dropping it, or a chicken coming out of it.)


For valid logic we must affirm the first part in order to deduce the second. In the fallacy we affirm the second part in an attempt to deduce the first. Affirming the consequent is fallacious because an event can be produced by different causes. Seeing the event, we cannot be certain that only one particular cause was involved.


If the Chinese wanted peace, they would favour cultural and sporting exchanges. Since they do support these exchanges, we know they want peace.

(Maybe. This conclusion might be the most plausible, but there could be other, more ominous reasons for their support of international exchanges. The cat can be killed in more ways than one.)


This fallacy receives a plentiful airing in our law-courts, since it is the basis of circumstantial evidence. Where we have no eyewitness evidence, we work back from what is known to those actions which might have caused it.


If he had been planning murder, he would have taken out extra insurance on his wife. He did take out extra insurance.

If he intended poison, he would have bought some. He did buy some weedkiller.

If he had wanted to cut up the body, he would have needed a big saw. Such a saw was found in his tool shed.

(There could be alternative explanations, innocent ones, for all of these actions. It would be fallacious to say that any of them proved him guilty. But as they mount up, it becomes progressively easier for twelve good persons and true to eliminate reasonable doubts about coincidence. No doubt they are sometimes wrong and thereby has hanged many a tale, together with the occasional innocent man.)


This is an extremely good fallacy to use when you wish to impute base motives to someone. Motives do not show, but the actions caused by motives do. You can always gain a hearing for your suggestion of less-than-honourable motives, by use of a skilfully affirmed consequent.


She’s just a tramp. Girls like that always flaunt themselves before men, and she did appear at the office party wearing a dress that was practically transparent!

(We can all see through this one.)


The above article is from the book How To Win Every Argument by Madsen Pirie. The article is only for educational and informative purposes to explain and understand formal logic and logical fallacies. It is a great book, definitely worth a read!