Updated: Jun 6
An investigation into proximate and ultimate starting points
This the second part of our practical apologetics series.
Anyone familiar with street level presuppositional apologetics will be familiar with a few common short quips used by apologists: "How do you know your reasoning is valid?", "How do you know you can trust your senses?" and "Could you be a brain in a vat?". Why are they asking these questions and what are the implications for our answers to them? Although on surface level these questions might seem to be worthless, they reveal in an expert manner the heart of the human problem that we all seem to face .
The questions come down to how we know what we know and certainty. It touches on the manner in which humans were made to operate. How can we be sure that our cognitive faculties are generally reliable in order to sufficiently provide us with knowledge of the outside world?
The point in the Christian asking these questions is to try and expose the unbeliever's (morally) foolish attempt to rely on their own autonomous abilities rather than on God in their thinking, thereby destroying the possibility of gaining any meaningful knowledge, and of predication. In this article we'll take a look at a few ways of answering the question and compare it with the Christian answer.
The evolutionist's accounting for sense perception
The evolutionist worldview hold that man is product natural selection acting on random genetic mutations (modern synthesis). In this view of the world, man is nothing more than the product of a material universe and some laws of mathematics acting on matter. On this view, man has been "created", or rather, "selected" for only one purpose:
... Darwinian natural selection, which is the process that has brought all living things to be the way they are, is best seen at the gene level, is best seen as a process of differential survival among genes, and therefore living organisms and their bodies are best seen as machines programmed by the genes to propagate those very same genes. In that sense we are gene machines. But it is not intended to be at all a demeaning or belittling statement.
Richard Dawkins, The Selfish Gene
That's the view of man - a body that is best seen as a machine programmed by the genes to propagate those genes - according to naturalists. The purpose of life is reproduction. That's it. Alvin Plantinga (American analytic philosopher) better developed an argument against naturalism that bothered Darwin himself:
But then with me the horrid doubt always arises whether the convictions of man's mind, which has been developed from the mind of the lower animals, are of any value or at all trustworthy. Would any one trust in the convictions of a monkey's mind, if there are any convictions in such a mind?
Charles Darwin, To William Graham 3 July 1881
Plantinga's argument can be summarised as follows : If evolutionary naturalism is true, then it cannot be rationally be held as true as there is no rational reason to believe that this mechanism can provide us with reliable cognitive faculties. Given that we were selected to reproduce, natural selection will select the organism that has a reproductive advantage - hence, your cognitive faculties will delude you if it gives you a reproductive advantage.
We were not selected for scientific inference and problem solving. Any person that concludes, using their cognitive faculties that evolutionary naturalism is true, has defeated themselves. This is called a self-stultifying view.
But the problem goes deeper of than this. The evolutionist used his cognitive faculties to formulate the theory of evolution. The conclusion the evolutionist reaches thus invalidates assumed reliability of his cognitive faculties he used in order derive the theory. But what if the evolutionist recognises this, and abandons the theory because of this reason. Is he then justified in holding to the reliability of his sense perception? Well, he still has no non-arbitrary reason to believe in the reliability of his senses. So where can he go from here? Let's review a few options.
The general answer
A common answer that has been encountered in discussion with unbelievers (and sometimes believers) is the attempt to ground the trust in the general reliability of our cognitive faculties in their evidence of past reliability. Our senses have worked quite well in the past, so we can trust them going forward. Is this a sufficient answer? Indeed not. It's an arbitrary statement of the desired result.
The answer, although it sounds nice, simply states that "my senses work because, as we can see, my senses have worked". This issue is that you cannot evaluate whether your senses work without using your senses. This would be an arbitrary (fallacious) circle.
Moreover, even if the argument somehow succeeded, there is no reason to believe that because our senses have worked in the past, they will continue working in the future. This is similar to the problem of induction which has also plagued philosophers since the life and work of British philosopher David Hume. Just because something worked in the past, does not mean something will work in the future unless unless there is reason to expect continued uniformity in nature. 
Is there a way out from this?
The agnostic enters the conversation
The agnostic is characterised by a humble acknowledgement of their lack of having enough information to know anything for certain. Hence the agnostic (at least in this case) would not try to say anything explicitly about the world around them with absolute certainty. The agnostic would therefore humbly admit that he does not know why he trusts his cognitive faculties as an attempt out to the problem. But this is a cop-out (an instance of avoiding a commitment or responsibility) that when pressed further, falls apart quite quickly.
If the agnostic is not certain about anything, the question can rightfully be asked if the agnostic is certain that they're not certain about anything. There can only be three answers to the question: "yes","no" or "I'm not certain if I'm not certain". If the answer is "yes", they've refuted themselves. If the answer is "no" or "I'm not certain", the cycle repeats and the agnostic is stuck in an infinite regress. The agnostic therefore never reaches a foundational or ultimate justification for any truth claims they might make. At this point, the agnostic has disqualified themselves from any form of meaningful debate. Any word they say falls into a meaningless void and, if they were consistent with this view, would recognise that their words have no meaning whatsoever. Anything they say or think is unprovable and at best arbitrary assumptions. There's no use in trying to consistently engage from this point on.
The agnostic might attempt one final pushback by attempting to levy the same critique against their opponents. If the agnostic cannot know anything, that applies to their opponent too, right? This might sound like a reasonable assertion for the agnostic, but it fails for two reasons:
The agnostic is not supposed to know anything. Hence, the assertion that their opponent cannot know anything is, like all other claims the agnostic makes, arbitrary and unprovable.
The assertion begs the question against the opponent's (let's say Christian) worldview. The Christian claims that we can and do have knowledge (which is even evidenced by the agnostic although they would self-stultifyingly deny it). The agnostic cannot on non-Christian ground levy an argument against the possibility of knowledge in the Christian worldview. 
It must be noted that different forms of realism has been adopted by many Christians, however it is the position of the writer that these positions (although they contain elements of truth) are ultimately destructive to the pursuit of knowledge if not placed squarely on Christian grounds.
The position of the classical realists can be summarised as follows:
All knowledge begins in the senses and is completed in the intellect.
A few years ago I participated in a public discussion on classical vs presuppositional apologetics. Unfortunately, I was not as well read back than as I am now (though I still have much to learn). When I asked the question of how my non-presuppositionalist friend come to know the external world, he said something that greatly echoes what Richard Howe recently wrote on his blog:
As humans, we encounter the existing sensible things of the external world by means of the senses. That world is reality (though, as Christians, we know that’s not all there is in reality). It is incoherent to demand that the knower somehow get back behind the sensible reality we know, and from there put together some “proof” or “argument” which arrives at the conclusion that there’s an external world out there that he knows. To put it more facetiously, if the world of sensible objects right in front of me is not enough for me to know that it’s there, then how can any argument about the sensible world be more compelling? The argument is itself one step removed from the world.
Richard Howe, How do I know what I know?
Notice that this argument can be used by the evolutionist and the agnostic alike (which is unfortunate that the classical apologist would give this Christian ground to the unbeliever). By simply stating that the world of the sensible things (that is, the things we perceive by our senses) is the world of reality, does not answer the question of how we can know that we're not a brain in a vat. It simply begs the question.
I know that the world of my sensible experience is the world of reality because ...
... the world of my sensible experience is the world of reality ...
It simply arbitrarily presupposes that we're not a brain in a vat and that's it. This is an arbitrary assumption and hence destroys the possibility of knowledge. Jason Lisle answered this quite well in a two-part blog post that can be found here and here.
There is no reason that keeps us from simply assuming the opposite that the realist assumes and there would be nothing the realist can do to refute the critic. This is not to say that the realist is incorrect in his assumption on the reliability of sense perception, but just that the assumption is arbitrary and therefore cannot ultimately ground knowledge. The realist at best finds himself in a Mexican stand-off with the supposed anti-realist.