Affirming the consequent is a formal logical fallacy that takes a true statement and invalidly infers its converse.
It is a formal logical fallacy because the fallacy is caused by a structural error in a deductive argument (an argument in which if the premises are true the conclusion is guaranteed to be true). This means that regardless of the content of the argument, it is the way it is laid out (structured) which causes it to be a fallacy.
The structure of affirming the consequent is a syllogism (two premises and a conclusion):
- Premise 1: If X then Y
- Premise 2: Y
- Conclusion 3: Therefore X
In the above argument structure 1 states that if X happens then Y will happen. But NOT that if Y happens X will happen. Therefore, when 2 and 3 conclude that Y makes X happen, it is not supported by 1 and therefore is a formal logical fallacy (the structure of the premises do not support the conclusion).
It is vital to note here that affirming the consequent is an incorrect version of a valid formal logical argument called Modus Ponens:
- Premise 1: If X then Y
- Premise 2: X
- Conclusion 3: Therefore Y
In the structure of Modus Ponens we can see that premises 1 and 2 support the conclusion 3.
Affirming The Consequent Fallacy Examples
1. Fun in the sun
“Even though the Sun seems to always be there, we need to feel its rays directly in order to produce vitamin D. If you are getting enough vitamin D then it’s safe to say you will be feeling healthy. Whenever I see someone looking healthy, I always think to myself they must be getting enough vitamin D.”
The above argument commits the affirming the consequent fallacy by claiming that when you see someone healthy it must mean they are getting lots of vitamin D. This is unsupported by the premise in the first sentence which only tells us that if you are getting enough Vitamin D then you will be healthy and not the other way around.
2. Hero to zero
“Celebrating early in sporting events has cost many athletes a first-place finish. Take Lindsey Jacobellis at the 2006 Winter Olympics. She started celebrating early and ended up falling, with another athlete beating her to the gold because of it. I think it’s fair to say that if you celebrate early, then you are going to end up falling and losing your place. A few years later at the next Olympics I saw a sprinter falling just before the finishing line. They must have been celebrating early. It really is such a silly mistake!”
This argument is an affirming the consequent fallacy because premise 2 claims that a sprinter fell and therefore concludes that they must have been celebrating early. However, this is not supported by premise 1, which only tells us that if you celebrate early you will fall. It does not say that if you fall you have celebrated early.
3. Take a deep breath
“The basic laws of nature tell us that a byproduct of photosynthesis is the presence of oxygen in the atmosphere. It’s thanks to this fact that scientists think we have any life on earth at all! Now that we are looking for new forms of life on other planets we can look for oxygen, because if there is life on a planet ,then, there will be oxygen as a byproduct. I can’t wait until they find oxygen on a planet! There will definitely be life there!”
In the above scenario the conclusion is that if we find oxygen we will find life on another planet. This conclusion is not supported by the first premise which states that if there is life there will be oxygen. Not that if there is oxygen there will be life. Therefore, this is an affirming the consequent fallacy
4. A job well done!
Alex and Imran are work colleagues. They both know that if they do well on a project then they will get a raise. The next Monday Imran walks up to Alex and excitedly tells him that they got a raise! Alex is overjoyed and slaps Imran on the back saying: “We must have aced that project then!”
Alex is committing the affirming the consequent fallacy by assuming that a raise means they did well on their work project. The first premise in this scenario is that if they do well on their project then they will get a raise. It does not say that the opposite is true. Therefore, Alex cannot conclude that they have aced the project when they get a raise because this is not supported by the premises.
5. A good student.
The best advice you could give to a first-year university student is to work hard. If you do well at university, then you will do even better once you have graduated. My old friend Stan did great after university, he must have worked hard as a student.
According to the structure of the above argument, if a person is successful at university they will be successful afterwards. When we are given information about Stan all we know is that he ‘did great after university’. The conclusion that he worked hard as a student does not follow from this information. It is therefore an affirming the consequent fallacy
6. Family feud
Every Christmas it’s the same story in the Jameson family. If Uncle John and Kenny start talking politics there will be a big fight. On the Christmas of 2021 there was a big fight, it must have been because of Uncle John and Kenny.
The conclusion of this scenario is not supported by the premise. The premise tells us that if Uncle John and Kenny talk politics then there will be a fight. It does not tell us that if there is a fight in the family it is because they were talking politics. Therefore, this is an affirming the consequent fallacy.
7. Just another rainy day.
Everyone knows that if it rains then the streets get wet. On Tuesday I went outside and almost slipped in a puddle right outside my house! It must have rained and I did not notice it.
In this scenario the first premise is that if it rains the streets will be wet. The next premise is that the streets were wet outside my house and therefore it must have rained. This is an affirming the consequent fallacy.
The reason it is a fallacy is because premise 1 does not support premise 2. It is true that the most likely explanation for why the streets would be wet is that it had rained. However, we are looking for errors in argumentative structure. The two premises must guarantee the truth of the conclusion, and in this case they do not, they only give probable reasons.
Some hawks in Australia and other places have been reported spreading wildfires in order to flush out prey for them to eat. It’s a stunning new scientific discovery. If you ever see a wildfire somewhere, then it could have been started by a hawk! After hearing this interesting fact, two helpful citizens in Australia called the fire department when they saw two hawks flying over a forest.
In the above scenario the two helpful citizens are committing the affirming the consequent fallacy. The premise in the first paragraph tells us that if there is a fire hawks could spread it even more. It does not tell us that if you see hawks there will be a fire nearby.
9. 3rd rock from the sun.
People have been prophesizing the end of the world – or humanity – for thousands of years. One way humanity could die out is if the sun stopped functioning, then we would surely go extinct. If we all go extinct, it must have been because our sun stopped working.
The conclusion that our extinction must come from the sun having stopped working is an affirming the consequent fallacy. There are many reasons the human race could go extinct. All we are told in this scenario is that if the sun stops working we will go extinct, this does not support the conclusion which is the opposite of this.
10. The secrets to success
Building good habits can be so important in order to lead a successful life. If you make something a habit, you do not have to worry about how or when you will do it, you just will. Whenever you see someone successful, you just know they have good habits.
The above is an affirming the consequent fallacy. The conclusion that successful people have good habits is not supported by the premise that good habits can be important in leading a successful life.
The key with affirming the consequent fallacy is to always remember it is a formal logical fallacy and so the structure of the argument is the thing to focus on. The two premises must guarantee that the conclusion is true no matter what. It can leave no room for doubt or alternative answers.
Whether the content of the argument is correct is not important when dealing with formal logical arguments. It’s easy to get side tracked by this fact. Just because an argument may be filled with correct facts such as: zebras have black and white stripes does not mean that the structure of the argument is correct.
Thinking about formal logic and their fallacies is a unique and important way to analyse arguments in academic tasks. It is also sometimes much easier to check if an argument is false by its structure than having to fact check every piece of information in the argument.
Dr. Chris Drew is the founder of the Helpful Professor. He holds a PhD in education and has published over 20 articles in scholarly journals. He is the former editor of the Journal of Learning Development in Higher Education. [Image Descriptor: Photo of Chris]