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Fallacy Friday: Petitio Principii (Begging the Question)

March 19th, 2011 by Matt

For the last few Fridays I have been publishing a blog series on logical fallacies. This begs the question, what fallacy will I discuss today?

The above sentence illustrates how the  phrase “begs the question” is commonly misused. To test how pervasive this misuse is I typed “this begs the question” into Google and limited it to New Zealand; every single result on the first page was a misuse and after delving eleven pages into the results I gave up – there was not a single correct use of ‘begging the question’ among the lot! For this reason it is important to begin this post by clearly setting aside what begging the question is not.

In my opening sentence I used the phrase “this begs the question” as a synonym for “this raises the question”. This is the way the phrase is commonly misused but this is not what the phrase actually means (something that  is a source of humour and irritation for English pedants).

The Fallacy of Begging the Question
“Begging the question” refers to the informal fallacy known as petitio principii, which literally means “requesting first principles.” The “question” in “begging the question” refers to the matter at the heart of the debate, the issue being debated. To “beg the question” is to attempt to have that question conceded by assuming it either implicitly or explicitly in the premises of the argument the arguer offers for its truth. In other words, the arguer assumes what he trying to prove and uses that assumption to prove that assumption correct.

Strictly speaking, an argument that begs the question is valid. In Analysing Arguments, I explained that a valid argument is one where it is impossible for the premises to be true and the conclusion false. Consider the following argument form.

[1] P


[2] P

This argument is pretty obviously a case of begging the question. The conclusion [2] is explicitly stated in the premise [1]. It is also the case that it is impossible for [1] to be true and the conclusion [2] to be false – they utter the same proposition after all. The same would be true of any argument that contains the conclusion in the premises. It will be impossible for the premises to be true and the conclusion false given that they are the same. Further, if the conclusion is true, and P is the case, this argument is sound; [1]-[2] is a valid argument with a true premise.

Despite this, there is obviously something fishy about these kinds of arguments and this is due to the function arguments play. Arguments are supposed to provide people with reasons as to why they should embrace a conclusion. A question begging argument relies on one already accepting the conclusion in order to provide one with reasons for doing so.

Given this, in any normal argumentative context question begging arguments are useless. No person who does not already hold the conclusion has any reason to validly infer the conclusion is true. To anyone doubting a proposition, appealing to that proposition in an argument to convince them of its truth is not going to get the arguer very far (unless the hearer falls for the fallacy).

Common Use
Arguments like [1] – [2] are pretty obvious examples of begging the question but there are less obvious, more subtle and more common versions of this fallacy. Often begging the question occurs when an argument has implicit premises; the arguer’s argument or the premises of his argument relies on some unstated assumption.  Because the assumption is implicit people do not notice it.

Circular ReasoningBegging the question is a form of circular reasoning where the inference circles from the conclusion to some proposition inferred from the conclusion and then from this proposition as a premise back to the original conclusion. In text books perhaps the most common example of circular reasoning is the following argument for the existence of God:

[1] The Bible is the word of God;

[2] What God says is true;

[3] The Bible says God exists;


[4] God exists.

This is obviously bad reasoning. Premise [1] assumes the conclusion in [4], that God exists. If God does not actually exist then he cannot speak through human texts. But the claim that God exists is both part of the first premise and the conclusion and is what the argument is seeking to prove.

This argument is widely derided (and justifiably so). What is less noted are the other arguments that are logically very similar, which are sometimes used by non-religious people to defend sources of information they consider reliable or authoritative.

Consider the argument that we can trust the findings of empirical studies because in the past such studies have proved to be reliable. Typically, when unpacked, this argument uses empirical studies of the past to show empirical studies of the present and future can be relied on.

Or consider the claim sometimes made in debates over the argument from religious experience. This is the claim that one can trust the five senses but not religious experiences because the former can be subject to empirical testing and the latter cannot. The problem is that “empirical testing” involves testing with the senses – you have to see, hear, touch, etc the results. If this is the reason why we know the former is reliable then we are, in fact, relying on our senses to prove that our sense are reliable.

Similar things can be said about rationalists who offer us carefully constructed arguments as to why we should trust reason. By engaging in arguments they are relying on reason to show that reason is reliable.

Now I am not intending to denigrate reason or trusting our senses; nor am I claiming that empirical studies are unreliable. The point is that these things cannot be proven to be reliable on the basis of arguments like these. I am inclined to think that with things such as our senses and reason, we must simply trust these sources as reliable. We cannot believe in their reliability on the basis of argument, neither can we prove they are reliable by argument. Even if I am wrong, any proof of their reliability cannot begin by implicitly assuming that they are reliable and then reasoning from there.

Fallacies like begging the question can be beguiling; the fallacious nature of such arguments is very obvious. However, the way our beliefs are related or the tacit assumptions we make in our day to day life or even the presuppositions we interpret reality through are often not immediately obvious to us without reflection. This is especially the case when these assumptions are taken for granted and not seriously challenged in the environment in which we live. For this reason when confronted with a position radically different from their own, people will often dismiss that position on grounds which seem plausible to them and to those who share their presuppositions without realising that, in some instances, it is these very presuppositions that are at issue in the first place.

Every Friday I publish another post in my Fallacy Friday series. To navigate the whole series, use the  tag.

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7 responses so far ↓

  • What do you think of John Frame’s discussion on pressupositional apologetics and circular reasoning?

  • I love having a Page Rank 6 blog 🙂

    “I typed “this begs the question” into Google and limited it to New Zealand; every single result on the first page was a misuse and after delving eleven pages into the results I gave up – there was not a single correct use of ‘begging the question’ among the lot!”

    Try it again – the very first hit now uses it correctly 😉

  • Give up. The meaning of “begs the question” has changed and it now means “raises the question”. I’m cool with that; there’s no point being pedantic when clear communication is your aim.

  • I don’t think that pre-suppositional apologetics necessarily beg the question. They simply look at what people pre-suppose when they make an argument and then try to show that this pre-supposition actually makes it probable that God exists.

    For example, Cornelius van Til’s so called “transcendental argument” for the existence of God essentially argues that the possibility of logic pre-supposes a theistic worldview. That’s not obviously circular

  • Yes – I often use the phrase “begs the question” to mean “raises the question”. This seems to be the common usage of this expression these days.

  • Similar instances of that:

    People often claim that a false proposition is a “fallacy”. It pisses me off when people say that sort of thing. I want to yell at them that “a proposition can’t be fallacious!!”

    Or similarly, people will say that a proposition is “valid” or an argument is “true”. It really pains my ears to hear people talking like that!

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