God establishes reason, and without Him, we have no reason to be reasonable.
We reason by the laws of logic combined with facts we observe. For example, the Law of Non-contradiction, that A cannot be both A and non-A at the same time and in the same relationship. These abstract, conceptual principles must be accounted for if any discussion on any topic is to take place. Only God can account for these laws. His thinking upholds ours.
If the laws of logic are based on human thinking, then we have to realize that people are different and the laws may differ from person to person. They are no longer absolute. Some particular examples follow.
If the laws of logic are just social conventions, then they are not absolute, and they can be ignored at will. My social network is, after all, different from yours.
How do we avoid the conclusion that all of our thinking is not the result of mere instinctive reactions to our environment? Since our environments are all different, we would all reach different conclusions. We would have no basis to argue because we could not change each other’s environments to the degree required to change each other’s minds.
If our thinking is a preconditioned response determined by our genetics, rational impulses would then be determined by genetics. There would be no decisions made in any traditional sense. We would all be pre-programmed to do what we do, and therefore there would be no sense in arguing. We could not change each other’s genetics, so no one could possibly win.
The laws of logic cannot come from science because science is based on inductive reasoning from things we see in our environments. For example, we cannot see the law of non-contradiction in the world. We would have to see the properties of a non-existent things (non-A). The laws of logic are abstract constructions that exist only in the mind. We discover the laws of logic by thought, not observation.
The laws of logic are also undeniable. If we say, for example, “I know the laws of logic do change. We’ve found scientific discoveries in quantum physics that disobey those laws.”
Any discovery you make must be logically understood. For example:
If the Law of Non-contradiction (A cannot be both A and Non-A at the same time, in the same relationship, and in the same sense) is not in effect, then you might as well say to me: “We’ve found scientific discoveries in quantum physics that obey the laws of logic.” “Obey” and “disobey” can have the same meaning in your sentence.
If the Law of Identity (something is what it is, something that exists has a specific nature) is not in effect, the discovery in question would not necessarily be a discovery. Your statement of the discovery could be: “We’ve found scientific discoveries in quantum physics that disobey those laws, but the discoveries changed into discoveries which followed the laws of logic when we came back the next day.”
If the Law of Excluded Middle (a statement is either true or false) is not in effect, your statement on quantum physics could be false even if it were true.
None of the sentences used by anyone would have any objective meaning if the laws of logic do not apply. If the laws of physics as they are currently understood do not apply, it does not follow that the laws of logic do not apply. This would go for any recent scientific discovery. As soon as a person opens their mouth to refute logic they have assumed it.
If Christians are wrong, we are left with no reason to think that we can have rational discourse. Atheists and agnostics cannot have an argument, because, in the ultimate sense, they do not have a position.
(Sources for the above are: John M. Frame, The Christian Apologetics and Research Ministry, and Tearing Down Strongholds: And Defending the Truth by R. C. Sproul, Jr. Of course, the mistakes are all mine.)