A post of mine ends like this:
To theists, I say: go on being theists. You are better off being a theist than not being one. Your position is rationally defensible and the alternatives are rationally rejectable. But don't fancy that you can prove the existence of God or the opposite. In the end you must decide how you will live and what you will believe.
About "Don't fancy that you can prove the existence of God or the opposite," Owen Anderson asks:
How would we know if that claim is itself true? Isn't it is possible that one or the other can indeed be proven?
To formulate my point in the declarative rather than the exhortative mood:
P. Neither the existence nor the nonexistence of God is provable.
How do I know (P) to be true? By reflection on the nature of proof. An argument is a proof if and only if it satisfies all of the following six requirements: it is deductive; valid in point of logical form; free of such informal fallacies as petitio principii; possesses a conclusion that is relevant to the premises; has premises each of which is true; has premises each of which is known to be true.
I say that an argument is a proof if and only it is rationally compelling, or rationally coercive. But an argument needn't be rationally compelling to be a more or less 'good argument,' one that renders its conclusion more or less rationally acceptable.
Now if my definition above gives what we ought to mean by 'proof,' then it is clear that neither the existence nor the nonexistence of God can be proven. Suppose you present a theistic or anti-theistic argument that satisfies the first five requirements. I will then ask how you know that the premises are true. Suppose one of your premises is that change is the conversion of potency into act. That is a plausible thing to maintain, but how do you know that it is true? How do you know that the general-ontological framework within which the proposition acquires its very sense, namely, Aristotelian metaphysics, is tenable? After all, there are alternative ways of understanding change. That there is change is a datum, a Moorean fact, but it would be an obvious mistake to confuse this datum with some theory about it, even if the theory is true. Suppose the theory is true. This still leaves us with the question of how we know it is. Besides, the notions of potency and act, substance and accident, form and matter, and all the rest of the Aristotelian conceptuality are murky and open to question. (For example, the notion of prime matter is a necessary ingredient in an Aristotelian understanding of substantial change, but the notion of materia prima is either incoherent or else not provably coherent.)
To take a second example, suppose I give a cosmological argument the starting point of which is the seemingly innocuous proposition that there are are contingent beings, and go on to argument that this starting point together with some auxiliary premises, entails the existence of God. How do I know that existnece can be predicated of concrete individuals? Great philosophers have denied it. Frege and Russell fanmously held that existence vannot be meaningfully predicated of individuals but only of cncepts and propositional functions. I have rather less famoulsy argued that the 'GFressellina' view' is mstaken, but this is a point of controversy. Furtrhertmore, if existence cannot be meaningfully predicated of individuals, how can individuals be said to exist contingently?
The Appeal to Further Arguments
If you tell me that the premises of your favorite argument can be known to be true on the basis of further arguments that take those premises as their conclusions, then I simply iterate my critical procedure: I run the first five tests above and if your arguments pass those, then I ask how you know that their premises are true. If you appeal to still further arguments, then you embark upon a vicious infinite regress.
The Appeal to Self-Evidence
If you tell me that the premises of your argument are self-evident, then I will point out that your and my subjective self-evidence is unavailing. It is self-evident to me that capital punishment is precisely what justice demands in certain cases. I'll die in the ditch for that one, and pronounce you morally obtuse to boot for not seeing it. But there are some who are intelligent, well-meaning, and sophisticated to whom this is not self-evident. They will charge with with moral obtuseness. Examples are easily multiplied. What is needed is objective, discussion-stopping, self-evidence. But then, how, in a given case, do you know that your evidence is indeed objective? All you can go on is how things seem to you. If it seems to you that it is is objectively the case that p, that boils down to: it seems to you that, etc., in which case your self-evidence is again merely subjective.
The Appeal to Authority
You may attempt to support the premises of your argument by an appeal to authority. Now many such appeals are justified. We rightly appeal to the authority of gunsmiths, orthopaedic surgeons, actuaries and other experts all the time, and quite sensibly. But such appeals are useless when it comes to PROOF. How do you know that your putative authority really is one, and even if he is, how do you know that he is eight in the present case? How do you know he is not lying to you well he tells you you need a new sere in your semi-auto pistol?
The Appeal to Revelation
This is the ultimate appeal to authority. Necessarily, if God reveals that p, then p! Again, useless for purposes of proof. See Josiah Royce and the Paradox of Revelation.
Move in a Circle?
If your argument falls afoul of petitio principii, that condemns it, and the diameter of the circle doesn't matter. A circle is a circle no matter its diameter.
Am I Setting the Bar Too High?
It seems to me I am setting it exactly where it belongs. After all we are talking about PROOF here and surely only arguments that generate knowledge count as proofs. But if an argument is to generate a known proposition, then its premises must be known, and not merely believed, or believed on good evidence, or assumed, etc.
"But aren't you assuming that knowledge entails certainty, or (if this is different) impossibility of mistake?" Yes I am assuming that. Argument here.
Can I Consistently Claim to Know that (P) is true?
Owen Anderson asked me how I know that (P) is true. I said I know it by reflection on the concept of proof. But that was too quick. Obviously I cannot consistently claim to know that (P) if knowledge entails certainty. For how do I know that my definition captures the essence of proof? How do I know that there is an essence of proof, or any essence of anything? What I want to say, of course, is that it is very reasonable to define 'proof' as I define it — absent some better definition — and that if one does so define it then it is clear that there are very few proofs, and, in particular, that there are no proofs of God or of the opposite.
"But then isn't it is possible that one or the other can indeed be proven?"
Yes, if one operates with a different, less rigorous, definition of 'proof.' But in philosophy we have and maintain high standards. So I say proof is PROOF (a tautological form of words that expresses a non-tautological proposition) and that we shouldn't use the word to refer to arguments that merely render their conclusions rationally acceptable.
Note also that if we retreat from the rationally compelling to the rationally acceptable, then both theism and atheism are rationally acceptable. I suspect that what Owen wants is a knock-down argument for the existence of God. But if that is what he wants, then he wants a proof in my sense of the world. If I am right, that is something very unreasonable to expect.
There is no getting around the need for a decision. In the end, after all the considerations pro et contra, you must decide what you will believe and how you will live.
Life is a venture and an adventure. You cannot live without risk. This is true not only in the material sphere, but also in the realm of ideas.