The following was adapted from Greg Bahnsen's Lecture titled, Van Tillian Apologetics, Part 2.
Bahnsen on Circular Reasoning
We want to remember, and not do as some Van Tillians have done, and just give the impression that circular reasoning is perfectly fine with us. We want to make very clear what kind of circular argument we are accepting. If someone says:
Your argument comes down to this: The Bible is true because the Bible is true!
That is not our argument at all. It will serve us good to listen to Bahnsen's debate with Gordon Stein, in fact at one point he accuses Bahnsen of this exact type of reasoning. Bahnsen never claimed that the Bible is true because the Bible is true, much less the Bible is true because the Bible says the Bible is true. This objection is also brought against us by the likes of RC Sproul and others.
Nevertheless, we do reason in a circle because the kind of method we use is based on the authority of God speaking in the Scripture.
The authority of God speaking in Scripture dictates the method that we use. And we use the method of argumentation to prove ultimately the authority of God speaking in the Scripture. So there is in a broad sense a circle, but another way of describing it is to say that this system of thought is internally consistent with itself. And all systems of thought has got to be internally consistent with themselves. The method we are using assumes the authority of Scripture and from there we prove the authority of Scripture, although it's not the logical fallacy of the Bible is true because the Bible is true, some people might still have issue with this because it has this broad circularity that does not seem acceptable to us. What do we propose as an alternative then? Are we rather to be autonomous? When we believe that we are autonomous, what method of reasoning do we use? Is human reason the final authority? When the person says that they appeal to reason, then haven't said anything at all, because reason can mean 35 different things easily. So the question then is, reasoning in what sense? Cartesian sense? Kantian sense? What does the unbeliever want to prove with the use of reason? That there is no God, and that humans are autonomous. So we say how his he going to prove that reason is the correct standard - what is he going to appeal to? Either:
If he appeals to something else, then reason isn't the ultimate authority. So the unbeliever is caught in the circle of reasoning on the basis of using reasoning as his final standard and proving that by appealing to autonomous reason. To this the unbeliever will answer that at least his system of thought is internally consistent. But our system of thought is internally consistent as well! We use God's Word as our final standard to show that God's Word should be our final standard. So we have two circles. Now we can't talk to each other? Of course we can. Let's assume that reason is the final authority, and you assume that God's Word is the final authority. Now if God's Word is the final authority all our problems can be resolved.
There are moral absolutes
There is a basis for logic
There is a basis for scientific induction
But if human reasoning is the final authority, then we must conclude that:
Logic is impossible
Science is impossible
Ethics is impossible
So yes we have our circles, but the unbeliever's circle, that is to say their worldview, destroys the possibility of knowing anything! And we can't claim that we don't know anything, as that itself is a knowledge claim, and hence the position of the unbeliever is absurd.
And so how do we know that the Christian circle (worldview) is true? What is the absolutely certain proof of God's existence according to Van Til? That without God you can't prove anything.
Jason Lisle on Circular Reasoning
The following was adapted from Jason Lisle's article titled, The Ultimate Standard
Jason Lisle writes the following:
Revelation is the giving of some of God’s thoughts to man. The question is how do you know that these thoughts are from God? How do you justify them? So the question I am going to ask you is, “How do you know that the Bible is revelation from God?” Can you answer this without any degree of circularity? Bahnsen says you can’t. For any answer you give will implicitly assume the truth of the Bible.
The essence of covenantal apologetics is that you must reason within the Christian circle or you cannot actually know anything. All beliefs would be ultimately unjustified apart from the truth of the Christian worldview because knowledge begins with God (Proverbs 1:7).
The petitio principii fallacy – is an argument that arbitrarily presupposes as a premise its own conclusion.
Circular reasoning can be done in a vicious way, or a virtuous way. If it is vicious, then it is also a petitio principii. On the other hand, if the premise is rationally necessary, then it is not arbitrary and is not a petitio principii. For example, arguing that laws of logic must exist because to argue against the laws of logic you need to use the laws of logic is a virtuous argument, but it is circular because it presupposes logic which is also its conclusion.
The way we demonstrate that the Bible really is revelation from God must entail a degree of circularity. We must presuppose this revelation in order to justify the preconditions of intelligibility by which we prove anything whatsoever, including the proposition that the Bible is revelation from God. The circularity is inescapable, and hence virtuous.
If it were not presupposed, we couldn't actually know anything at all.
Some Further Thoughts
Let take sense perception as another example before we close. How do we know that our senses are generally reliable? Some people might make their sense perception to be their ultimate standard, and hence like Bahnsen showed us, the person would have to justify his general reliability of sense perception either:
Using his senses
Or using something else.
If he uses something else, his senses would no longer be the ultimate authority. If he uses his senses to justify his senses, the unbeliever is again caught in the circle of justifying his senses based on his senses.
It it is not rationally necessary that our senses function correctly, indeed we know of people whose senses don't function correctly. Therefore, the circle of the unbeliever is an arbitrary circle and hence fallacious.
It also remains impossible to deduce things like ethics, laws of logic and scientific induction from our senses. It simply isn't possible, most probably because our senses don't mean anything apart from our reasoning, but Bahnsen already handled this above. So why isn't the Christian circle also arbitrary?
Because as Jason Lisle showed, it is rationally necessary. Just as we cannot argue against the existence of the laws of logic without using the laws of logic, so we can't argue against the Christian worldview without borrowing from the Christian worldview. We can't prove the Bible without the Bible, as without the Bible proof doesn't make sense.
The preceding is what is called a transcendental argument.
The proof of the Bible isn’t a vicious circle such as “The Bible is God’s Word because it says it is.” Rather, the proof is that the Bible says it is God’s Word and makes knowledge possible. This is something the Bible itself indicates (Proverbs 1:7, Colossians 2:3). The Bible proves itself and is the only basis for proving anything else. It’s not a simple (vicious) circle, but a spiral that goes beyond itself and justifies our other beliefs. The proof of the Bible is the impossibility of the contrary. If the Bible were not true, we couldn’t prove that anything is true.
Jason Lisle, The Ultimate Standard