Fallacy Watch: Affirming the Consequent
  • 3-minute read
  • 10th March 2021

Fallacy Watch: Affirming the Consequent

“Affirming the consequent” is a fallacy (i.e., a bad or faulty argument). And since fallacies are never desirable in academic writing, we’ve prepared this guide to help students and researchers avoid affirming the consequent in their work.

What Is Affirming the Consequent?

Affirming the consequent – otherwise known as a “converse error” – is a logical fallacy that involves taking a true statement and assuming the converse form would be true as well. Formally, we can represent this fallacy as follows:

If X is the case, then Y is also the case.

Y is true, so X must be true as well.

However, even if the first part of such an argument is true, the second part doesn’t necessarily follow as there may be other explanations for “Y” being true. Let’s look at a couple of simple converse errors to see how they work.

Examples of Affirming the Consequent

Let’s start with an obviously untrue example of affirming the consequent:

For a bird to be able to fly, it needs two wings.

Emus are birds with two wings, so emus can fly.

Here, it is hard to deny the first part: any bird with fewer than two wings would struggle to fly. But the second part is, thankfully given the size of an emu, untrue.

In this case, the problem is that having wings is necessary for a bird to fly, but it is not sufficient by itself. Flying birds are also usually lighter than an emu, and the wings of an emu are far too small to lift their bodies off the ground.

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Thus, while the first part of our bird-based example is true, the second part flips the claim while overlooking other factors that enable flight.

In other cases, it may be that alternative explanations are overlooked:

If you have the flu, you will experience a fever.

You have a fever today, so you must have the flu.

This might seem plausible. However, while a fever is a symptom of flu, it is also associated with other conditions. And because the second part of this example does not necessarily follow from the first part, it is fallacious.

How to Avoid the Fallacy

If you spot an argument like the ones set out above, approach it critically. Does one statement being true justify believing the converse statement? Or, like with our emu and flu examples, is there something you might have overlooked?

Keep in mind, too, that a conclusion drawn by affirming the consequent might be true. Having a fever, for instance, is a reason to suspect someone has flu. But you would need to consider other potential causes for the fever and find evidence to claim it as a fact. If you don’t do this and simply assert a truth based on the converse form being true, you will leave yourself open to errors.

If you take a critical approach to your own writing, on the other hand, you should be able to avoid affirming the consequent. And to make extra sure your arguments are clear, having your work proofread can help. Submit a free trial document today to find out more about how our academic proofreading services work.

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