False Assumption

written by Steven Lewis

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Years ago my dad and I were sitting in a restaurant when he wadded up a napkin and said to me, “I’ll bet you five dollars I can get this napkin into that trashcan,” pointing to a small can clear across the room. After considering the distance and the lousy aerodynamics of the wadded napkin, I judged the feat to be impossible. “You’re on,” I replied with confidence. He then proceeded to stand up from the table and walk across the room to the trashcan, where he dropped the napkin in with ease. I had been duped!

That day I learned an important lesson about the logical fallacy of the False Assumption.

A false assumption occurs whenever the truth of a premise depends on some necessary condition which is merely stipulated or assumed to be present when it is not in fact present at all. In other words, this fallacy assumes that whatever it takes to make a premise true is already there, even if it is really absent or unknown. As in the example above, the premise in question was that it was impossible for my dad to get the napkin in the trashcan, and I falsely assumed my dad was going to throw the napkin from his seat. I never stopped to consider that the method of delivery had never been discussed, and thus both my assumption and the premise that the feat was impossible both turned out to be false.

This common fallacy of the false assumption often overlaps with a number of other logical fallacies in subtle ways, and thus it is not always obvious and can be difficult to spot. In fact, some logic texts consider false or unwarranted assumptions as an inherent characteristic of a whole host of other logical fallacies.¹ However there are characteristics of this fallacy which make it stand out—specifically the reliance of a premise on prior unsupported or unstated assumptions that turn out to be false.

It’s simply part of human nature that we often tend to “fill in the blanks” and assume what isn’t there when limited information is given, and so we sometimes end up committing the fallacy without realizing it. In fact, some things like jokes, riddles, superstitions, conspiracy theories, and even some magic tricks thrive on false assumptions. For example, Peter Kreeft gives the following joke as an example of a false assumption fallacy:

        “When you sold me this parrot you told me it could repeat every word it heard.”

        “That’s right.”

        “Well, I’ve been talking to it all day and it hasn’t said a word yet. That’s false advertising.”

        “No it isn’t. The parrot is deaf.”²

To help illustrate some different ways that false assumptions can creep into an argument, I have included a few examples below. As you can see, the fallacy of the false assumption can crop up just about anywhere. Look at the examples below and see if you can spot the false assumptions!

Examples of the Fallacy

“You sold me a faulty computer! When I press the power button, nothing happens.” The false assumption here is that all the other conditions necessary for a computer to work are in place and functioning properly. In fact I may just be pressing the wrong button, or it might not even be plugged in!

“Peter said he’s been at home all day and never heard the doorbell, so the delivery man must not have come.” This assumes that Peter was in a position to hear the doorbell (maybe he was asleep or listening to his headphones), and it also assumes that the delivery man would have rung the doorbell if he had come.

“God will bless me with great riches because I asked him and said, ‘in Jesus name.’ After all, Jesus says, ‘If you ask Me anything in My name, I will do it’ (John 14:14).” This argument falsely assumes that this statement of Jesus is referring to simply saying the words “in Jesus name,” whereas it is actually referring to requests made according to His will and for the glory of God.

“It is clear that God can be affected and changed. Otherwise, there would be no way God could have a truly loving relationship with me.” The false assumption here is that a loving relationship always requires the capacity to be affected and changed, even when applied to an already-perfect being (such as God).

“What stands on four legs but never sits or walks?” This last example is not itself a logical fallacy (or even a logical premise for that matter). However, this question actually relies on the answerer to commit the false assumption fallacy. This is a common riddle that can be tricky for any who might falsely assume that “legs” must belong to an animal of some kind. In fact, the answer is not an animal at all—it is a table.

These are some examples as to how identifying erroneous assumptions in an argument can help to expose the fallacy and defeat the argument, but remember that not all assumptions are false or unwarranted. The trick is not to avoid assumptions altogether, but rather to be aware of your assumptions and determine whether they are true or just assumed to be true (and possibly false!).


How to Avoid the Fallacy

The fallacy of the false assumption is not only difficult to spot in others’ arguments, but it can also be difficult to spot in your own. Here are some tools that may help you avoid the fallacy of the false assumption.

First, be aware of your assumptions. Realize that we make assumptions all the time, and ask yourself if the premises of your argument are themselves based on observation, evidence, or sound arguments. If not, they may be based on unstated assumptions. Being able to identify and recognize your assumptions is often an important step in making good arguments.

Second, check your assumptions. Just because you may have identified an unstated or unsupported assumption does not mean that the assumption itself is false. Ask yourself whether the assumption is true and is itself supported by its own evidence or arguments. Why is it assumed in the first place? Are there good reasons to believe the assumption is true? Are there alternative explanations or possibilities that have the same (or better) explanatory scope as the assumption in question?

Third, remember that even if a premise is based on a false assumption, that does not necessarily mean that the premise itself is false. The premise may still be true for other valid reasons. For example, a sports fan may believe his team will win the big game because he wore his lucky socks, but even though his underlying assumption is false, it does not follow that his team is bound to lose. They may yet win, even though the socks have nothing to do with it.

Finally, remember that the best arguments are usually those that identify and defend their own assumptions. So if you find that some of your arguments may be resting on false or unwarranted assumptions, then either take the time to justify those assumptions or abandon those assumptions altogether and look for other evidence to support your premises.

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  1. For such an example, see T. Edward Damer, Attacking Faulty Reasoning: A Practical Guide to Fallacy-Free Arguments, 7th ed. (Boston: Wadsworth, 2013).
  2. Peter Kreeft, Socratic Logic: A Logic Text Using Socratic Method, Platonic Questions, and Aristotelian Principles, 3.1 ed., ed. Trent Dougherty (South Bend, IN: St Augustine’s Press, 2010), 100.






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