Is it reasonable to believe that which is true?

Updated: Jul 12, 2019

As with most of my blog posts the inspiration for this one comes from me watching, or in this case re-watching, a video on YouTube. In this case it was the first part of Matt Dillahunty's review of his 2014 debate with Christian apologist Sye Ten Bruggencate. In the debate Sye made the claim that "It is reasonable to believe that which is true" and stated that it is unlikely that anyone will disagree with him on this, which of course Matt rightly did. In this post I am going to run through Matt's reasons for doing so, as well as expanding on them a little and relating them back to what is rapidly becoming a series of posts on whether it is reasonable to accept things or not.


While it is not really relevant here, in the debate Sye used this claim as part of a syllogism designed to demonstrate that it is reasonable to believe that God exists. His argument went like this:


P1. It is reasonable to believe that which is true

P2. It is true that God exists

C. Therefore it's reasonable to believe that God exists


Now Sye fully expected Matt to disagree with premise 2, that it is true that God exists, but, as previously stated, he did not expect him to have any issue with premise 1. However, as we shall see, this first premise is not as strong or agreeable as Sye implies.


The implication that Sye is making here is that there is a implicit connection between truth and reasonableness. If, for example, it is true that 2 plus 2 equals 4, then, Sye argues, it is also reasonable for us to believe that 2 plus 2 equals 4. Likewise, if it is true that God exists then it follows that it is reasonable to believe that God exists. The problem is that, while this might intuitively seem correct, it is not the case that truth and reasonableness are necessarily connected. As Matt points out in his video "reasonableness is not related to whether a claim is true, but rather to whether we have good reasons to believe that it is true". Let's break this down shall we.


If I told you that there is a rock on Mars that looks like Bigfoot strolling across the Martian surface, would it be reasonable for you to believe me?

Bigfoot taken from the 1967 Patterson–Gimlin film

Let's say all you have to go on is the claim itself, is it reasonable to believe what I am telling you? Well, if we go back to The Grid of Reasonableness, we can see that what you are dealing with is a second hand account about an unusual claim. As such it is reasonable to accept what I am saying, as long as you apply a healthy amount of skepticism.


Ok, so now what if I showed you the picture below and explained that it was taken by the Mars Exploration Rover Spirit in 2007. You now have photographic evidence to back up the claim I am making, and as such you no longer just have to evaluate the reasonableness of what I am saying based upon the claim alone. So is it now more reasonable for you to believe what I am saying than it was when you had nothing to go on but the claim itself?

Bigfoot on Mars captured by the Mars Exploration Rover Spirit, 2007

I suspect all of you will agree that it is more reasonable to believe the claim backed up by evidence than just the claim alone. But what I am sure you will all have noticed is that at no point did the truth value of the claim change. It was true that there is a rock on Mars that looks like Bigfoot when you had nothing to go on but my say so, and it was equally true that there is a rock on Mars that looks like Bigfoot after I showed you the picture. Nothing about the truth of the statement changed when you were presented with evidence, but the level of reasonableness of you believing me did.


Let's now flip things around a bit and ask whether it is reasonable to believe something that is false. In his 2010 paper, Things that make things reasonable, published in the journal Philosophy and Phenomenological Research, John Gibbons tells the story of the Milk Thief. Imagine, he says, that before you go to bed one night you check the fridge and see that you have a large container of milk sitting there. You head off to your bedroom and go to sleep. In the middle of the night a thief breaks into your house. He is a master burglar and manages to get in without making any noise or tripping the alarm. He goes to your fridge, opens the door, and steals your milk. He then makes a clean getaway, without leaving even the smallest sign, save for the missing milk, that he was ever there at all. The next day you wake up and head into the kitchen for breakfast. Now here is the all important question.


Before you open the fridge, is it reasonable for you to believe that you have milk?


As I am sure you will agree, and as John Gibbons concludes, the answer is yes. The last time you checked you had milk. You have no reason to think that anything has changed, and as such it is entirely reasonable for you to believe you have milk. Furthermore, it is reasonable for you to believe this even though in reality it is factually untrue that you have any milk in the fridge. At this point Gibbons introduces the idea of the Milk Anti-Thief.


As you head to bed one night you check the fridge and see that you have no milk. You head to your bedroom and go to sleep, and in the middle of the night the Anti-Milk Thief arrives. As in the previous story he manages to break into your house completely unheard, only this time instead of stealing anything he instead places a four pint container of milk in your fridge. As before he makes a completely clean getaway. In the morning you once again come down for breakfast and we ask the same question again.


Before you open the fridge, is it reasonable for you to believe that you have milk?


Using the same reasoning as the previous time, that being that you checked the status of the milk the night before and have no reason to believe it has changed, then in this case it is not reasonable for you to believe you have milk. But of course it is actually true that you do have milk in the fridge. Again we see that the truth of a claim or situation does not necessarily relate to whether it is reasonable to believe something. Here we have two situations, one in which it is reasonable for you to believe something that is false, and another in which it is unreasonable for you to believe something that is true. So if it is not truth that makes believing something reasonable or not then what is it? Well Gibbons sums this up nicely.


"Whether a belief is reasonable or you have reason to believe doesn’t just depend on the facts. It depends on the evidence available to you."


This is something that Matt echoes in his video when he says the following:


"Whether or not a belief is reasonable is not determined merely by whether or not it is true, but by whether or not we have good reasons to believe that the claim is actually true."


So let's bring this back to the first premise of Sye's syllogism.


It is reasonable to believe that which is true


Well, as we have seen, this is actually incorrect. It can be reasonable to believe that which is true, but likewise it can be unreasonable to believe that which is true, and entirely reasonable to believe that which is false. Instead, as we have discovered, it is reasonable to believe that which we have good reasons to believe. Now, as I have stated in a previous post, what constitutes "good reasons" can chance based upon the nature of the claim, but in general I am happy to go along with Matt's rewording of this first premise:


It is reasonable to believe that which has been rationally demonstrated, by sound argument and accepted evidence, to be true


Unfortunately for Sye however, this more accurate rewording of his first premise means his syllogism now reads like this:


P1. It is reasonable to believe that which has been rationally demonstrated, by sound argument and accepted evidence, to be true

P2. It has been rationally demonstrated, by sound argument and accepted evidence, that God exists

C. Therefore, it is reasonable to believe that God exists


Well I for one am completely ok with that, and I expect I shall see the evidence in support of premise 2 any day now.




Bigfoot image taken from the 1967 Patterson–Gimlin film

Bigfoot on Mars picture care of NASA


Edited 12/07/19 16:43 to correct spelling mistakes

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