INTRODUCTION
Often we find atheists and, to a lesser extent, agnostics in the online debate sphere demand that the verification principle is the only rational way of establishing whether or not god(s) exist. I would like to counter that idea with an argument I think is both the most sound argument for god while also being one of the weakest, the argument that people have experienced god(s).
I know that when this is first seen that many people will be quick to judge that it is a weak argument, but I hope that we can take the time to go over the premises first. This argument can be broken down, ultimately, into two parts. Part One is establishing that using Reid's Principle of Credulity is rational. Part Two is showing short and to the point, applying it to god(s). I will also preemptively address a common criticism of the argument in a Part Three.
PART ONE
“In absence of counter-evidence — we should believe that things are as they seem to be.” - Richard Swinburne
The above quote comes from the philosopher Richard Swinburne, one of the main proponents of the argument for god from Reid's Principle of Credulity. It helps outline what, precisely, the principle is. It helps with establishing the position that one should tentatively hold to. This is why, despite thinking the argument is sound, I hold that it is a weak argument for the existence of god(s), as when it stands on its own any proper counter-evidence defeats it. This is also why I feel the argument is useful, as it forces the atheist and the agnostic to have to provide counter-evidence. No longer would the defense of "lack of belief due to no evidence" be justified. One of the strengths of the argument, in my mind, is that we all already make use of this principle in our day-to-day lives, even if we aren't really aware of it.
In this part I will go over three different premises and how they all lead to the principle's validation.
P1) Your senses tend to be accurate.
P2) You are honest more often than you are dishonest.
C1) If you claimed to have experienced X then, absent counter-evidence, you likely had a true experience of X.
P3) There is no justification that the above is not universal.
C2) If someone claims to have experienced X then, absent counter-evidence, they likely had a true experience of X.
The strength of this argument is that denial of P1 or P2 will lead to different issues while P3's denial requires the use of the special pleading fallacy. To avoid fallacious logic and avoid logical traps you must accept all three premises, which means accepting the conclusion of the argument. To explain,
Rejection of P1 leads to solipsism, as if your senses tend to be inaccurate or are only accurate half the time then you will never be justified believing anything you observe is true. How, then, can you hold any justified beliefs about the external world? You cannot, as any piece of evidence cannot be justifiably believed.
As such, if you wish to avoid solipsism you must accept P1. This means that your senses tend to be accurate, which results in the conclusion that you should believe your senses unless you have a justified reason to doubt (counter-evidence).
Rejection of P2 also is a trap. If you reject P2 you must either choose the view that you are dishonest more often than you are honest or that you are dishonest and honest an equal amount.
If you reject it by claiming "you are dishonest more than you are honest" then it leads to the conclusion that we cannot trust that any statement you make is true without corroborating evidence, but that includes your statement that "you are dishonest more than you are honest." Since we cannot trust that that statement was made honestly, it is justified to think that you do not actually believe that "you are dishonest more than you are honest".
Even claiming to be honest and dishonest evenly does not avoid this issue, as it means we must suspend judgement on any statement you make until there is reason to accept, including whether you are honest and dishonest evenly.
We can have a thought experiment as well, is this a case you are being honest or dishonest? If dishonest, then we now know that you are either honest more or less often than dishonest, which goes back to the previous point where we end up unjustified thinking that you are dishonest more than you are honest. Since that is justified and the thought experiment is that your statement about being honest and dishonest evenly is false, that means we can conclude that you are, in fact, honest more often than dishonest (and that this dishonesty happens to just be an example of the minority of statements you make).
If, in our thought experiment, we assume that the statement is held in true belief then it leads to the conclusion that (absent corroborating evidence) we are unjustified in actually believing it is held in true belief.
This means that the only justified view is that, regardless of what you say, you actually accept P2.
Furthermore, rejecting P2 is not at all pragmatic when it comes to living your daily life. If you reject P2 and do not live your life as if you reject it, it raises doubts on if you actually reject P2. As people tend to live pragmatic lives, then unless we have corroborating evidence, we are yet again justified in thinking that you actually accept P2 regardless of what you say.
A consequence of P2 is that what you say should be believed to have been said with honesty unless we have reason to believe otherwise (counter-evidence).
So, since your senses tend to be accurate (which results in the conclusion that you should believe your senses) and you tend to be honest (which means we should trust your statements to be made honestly unless we have reason to believe otherwise), we can conclude that "If you claimed to have experienced X then, absent counter-evidence, you likely had a true experience of X".
Since we cannot make use of the special pleading fallacy to say that this is somehow a unique characteristic of you, we must extend this universally. This means that we can conclude that "If someone claims to have experienced X then, absent counter-evidence, they likely had a true experience of X".
This means we have validated Reid's Principle of Credulity.
PART TWO
This is a short segment that has one premise that should be uncontroversial,
P4) People have claimed to have had an experience of god(s).
C3) Absent of counter-evidence, these people likely had a true experience of god(s).
If part 1 is sound and P4 is true, then C3 must be true. From here it is a question on if the atheist or agnostic can provide counter-evidence. If none can be provided then the logical conclusion is that we should tentatively hold to theism as true.
PART THREE
"Extraordinary claims require extraordinary evidence" - Carl Sagan
The above quote from Carl Sagan is not uncommon in the online religious debate sphere, where the god claim is often called an extraordinary claim. A common rejection I have seen to the above is that Reid's Principle of Credulity might be evidence, but it isn't extraordinary evidence and thus cannot be used to argue for the existence of god(s) (as the god claim is labeled as extraordinary).
I would contest that this argument actually works. In fact, I believe that in order to justify the view that the god claim is an extraordinary one would require you to provide evidence/reasoning that would already act as the counter-evidence that the argument already accounts for. To explain, in order to say a claim is extraordinary rather than ordinary you must either draw the line arbitrarily (which makes the label unjustified), appeal to something internal to the claim that makes it extraordinary, or appeal to something external to the claim that makes it extraordinary.
I would argue that you cannot point to something internal to the claim that makes it extraordinary. This is because if the justification was internal to the claim, then the context of when or where the claim is made does not change whether it is ordinary or extraordinary. This just is not the case, however. For example, if I say I will be visiting South Korea in a few days and will be back in just over a week that is an ordinary claim today, but 200 years ago that would be extraordinary. Another example, let's say that I saw a floating city. Extraordinary claim, right? But let's say the same exact claim was made in a stereotypical high fantasy world, not that extraordinary anymore.
What this means is that justification for the label "extraordinary claim" can only be something outside of the claim itself. Whatever you appeal to would constitute evidence that said claim is false or unlikely. This means that it isn't that the claim is extraordinary, it is just that the claim already has counter-evidence. In order to justify the label of a claim being extraordinary you will need to provide that counter-evidence, but in doing so you are already operating in-line with the argument (as it only concludes that the claim should be held in absence of counter-evidence).
I want to address one more quote by Carl Sagan as well,
"Absence of evidence is not evidence of absence" - Carl Sagan
This is one I absolutely agree with. Absence of evidence alone is not evidence of absence unless you can construct a proper modus tollens. This means that, unless you provide a sound modus tollens, you cannot appeal to "lack of evidence" (using whatever criteria you give as to what evidence is) as being justification for the claim being an extraordinary one. Lack of evidence is not counter-evidence, it is only a lack.
CONCLUSION
With this argument it ends up with the atheist or agnostic needing to now provide justification for their position in the debate. Counter-evidence needs to be provided or else theism is rationally concluded as true (tentatively). Not only must counter-evidence be provided, but they also must defend against arguments that it is not, in fact, counter-evidence. That is the strength of this argument, it makes it so no longer is the theist playing defense but the atheist and agnostic is.
I know this argument will be unconvincing to most people, but it is important to remember that an argument can both be sound and also be unconvincing. We are not purely rational beings after all.