Practical apologetics #2: How do I know what I know?

Updated: Mar 23

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 [1].

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 [2]: 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. [3]

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:

  1. 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.

  2. 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. [4]


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.

  1. I know that the world of my sensible experience is the world of reality because ...

  2. ... the world of my sensible experience is the world of reality ...

  3. (repeat).

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.

Completed in the intellect

The realists might fight back with one more blow. The classical realists don't merely believe that all knowledge come from sense experience, but that the sense experience is completed / perfected in the intellect.

In my view this doesn't improve the situation, as the position still states that nothing is in the mind that was not first the senses. This means the assumption that the perception of the senses correspond to the actual world out there still needs to be made.

The realist has no way to know that the world of the senses actually corresponds to the external world.

The general problem

Whatever your philosophy, the general problem remains - how do you know your senses and reasoning is valid? There doesn't seem to be a way around this. This has been captured by something called the Münchhausen trilemma. The trilemma goes as follows:

There are only three options when providing further proof in response to further questioning:

  • The circular argument, in which the proof of some proposition is supported only by that proposition (my cognitive faculties work because they work).

  • The regressive argument, in which each proof requires a further proof, ad infinitum. (my cognitive faculties work because of A, A is the case because B, B is the case because C ... and so on and so forth).

  • The dogmatic argument, which rests on accepted precepts which are merely asserted rather than defended (my cognitive faculties work because A. A is simply the case and that's it - as in the case of the realist).

We've seen variations of the above three options in our discussions above. So, where does this lead us? Is all knowledge at heart arbitrary? How is it possible for us to know something for certain (as we clearly do know some things for certain)?

Jason Lisle notes that if the Münchhausen trilemma is correct, then we can demonstrate that knowledge is impossible. But, of course, this is instantly self-refuting. Because, is the truthfulness of the trilemma itself not something that must be known in order to be rationally affirmed? How do we escape these issues?

The heart of the issue

All of the above discussed attempted solutions, nicely summarised by the Münchhausen trilemma, all have one thing in common: They assume the ultimacy of man. The issue is that no matter how complex a solution the philosophers can think of, there is no way to escape the trilemma if we continue thinking about man is the final or ultimate reference point of knowledge. We would be forever stuck in the egocentric predicament [5] with the inevitable result of a self-refuting solipsism.

The answer to the above problem lies in the truth of the Christian worldview. We can know things for certain because the Christian worldview is true. That is the full answer. All non-Christian answers with an autonomous epistemology are fallaciously circular, absurd and an infinite regress of question begging [6] because they assume that man is the ultimate starting point. The view of man is that of a blank slate that needs to start constructing a worldview that is ultimately the product of their egocentric experience.

The justification in the Christian worldview does not succumb to the trilemma because it is not arbitrary, it does not form a regressive argument nor is it a dogmatic assertion, and because it does not assume man as the ultimate starting point, but God. We'll expand on this below.

The Christian answer (and some objections anticipated)

Jason Lisle summarises the Christian position as follows:

The [Christian] can answer this [reliability of senses] by pointing to the biblical worldview in which God has designed our senses to inform us of the external world, and God is a God of truth, not of deception.

Jason Lisle, How do I Know that I Know? – a Response (Part 1)

And in another article:

As a [Christian], my answer to this question is: by revelation from God. God has revealed Himself in such a way that there is literally no excuse for denying that what we observe is indeed His creation (Romans 1:18-20). God made our senses, and since God is truth, our senses will have some degree of reliability.

Jason Lisle, How do I Know that I Know? – a Response (Part 2)

Whereas the classical realists would argue that all knowledge begins in the senses and is completed in the intellect, the presuppositionalist will argue that the fear of the Lord is the beginning of knowledge (Proverbs 1:7), that we should have a philosophy that is according to Christ and not according the elements of this world (Colossians 2:8), and that it is in God's light that we see light (Psalm 36:9).

This is consistent with the Christian's view that there can be no neutral ground between the Christian and the non-Christian worldview. There is either submission to Christ from the start of the reasoning process, or there isn't. You either treat the fear of the Lord as the beginning of knowledge, or as the culmination of knowledge. John Frame writes the following:

God’s word is the ultimate criterion of certainty. What God says must be true, for, as the letter to the Hebrews says, it is impossible for God to lie (Heb. 6:18, compare Tit. 1:2, 1 John 2:27). His Word is Truth (John 17:17, compare Ps. 33:4, 119:160). So God’s word is the criterion by which we can measure all other sources of knowledge.

Now this sounds good on paper, but didn't we have to read the Bible in order to understand the Bible? Doesn't this also beg the question that our senses and reasoning are reliable when we read the Bible that told us that our senses and reasoning are reliable?

An arbitrary argument for the reliability of our senses would look something like this:

  1. I assume my senses are valid because ...

  2. ... the Bible says my senses are valid, which I can read and understand because

  3. (repeat). I assume my sense are valid because ... (and so on and so forth).

So what's going on here? What is Lisle and Frame getting it? How can the Christian escape the arbitrariness pointed out in the other philosophies? The answer lies in the fact that the above argument still assumes the ultimacy of man, and not of God. That's not the Christian argument, and that's not what Lisle and Frame's answers entail.

Don't I have to assume my reason / senses are reliable before reading the Bible?

The short answer to the sub-heading is a plain yes. We had to assume that our senses were reliable before reading the Bible to know what the Bible says about reality. But this is where we should make the important distinction between proximate starting points and ultimate commitments, and / or temporal and logical starting points for reasoning. Van Til describes the issue as follows:

... If then the human consciousness must, in the nature of the case, always be the proximate starting-point, it remains true that God is always the most basic and therefore the ultimate or final reference point in human interpretation.

Van Til, The Defense of the Faith