There is an argument that one sometimes encounters, typically from those who deploy presuppositional apologetics, that goes by the name of the “unity of knowledge” argument. I will not be fully addressing this argument here, so I shall commit the grave academic sin of not properly stating this argument in its best form–and as such, I invite the reader not to take what I say here as necessarily a refutation of this argument. That disclaimer given, the broad strokes of the argument is that: no-one can have any knowledge without knowing that global scepticism is false, which means either being omniscient or having access to an infallible omniscient source, therefore God exists. To distill this into a question that I will give my answer to (or at least, an answer I lean towards) in this article, we ask:

How can the atheist overcome global scepticism–i.e. how can they believe that knowledge is possible?

Scepticism and the unity of knowledge

What is sceptic’s challenge? The sceptic challenges one’s belief that “Knowledge is possible”. Let us call this belief KP.

Why is KP so important? After all, it is not a matter we think about very often. Those who have never studied epistemology may well spend their whole life never considering the matter. However, there is good reason to think we rely on this belief at all times. Consider for instance the argument, to which we will return later:

1. If one believes that p, then one believes one knows that p (premise)
2. If one knows that p, then this transparently entails KP (premise, obvious)
3. Therefore, if one believes that p, then one believes that KP

So if it turned out that one’s belief that KP was undermined, that would infect all our other beliefs. I take this to be the heart of the “unity of knowledge” argument: all knowledge presupposes KP.

How does the sceptic challenge KP? Typically, they devise some sceptical scenario S (e.g. you are being deceived by an evil demon, or are a brain-in-a-vat etc.). Let us here take S to be the disjunction of all such sceptical scenarios. Then they argue that:

(SP) If S, then ¬KP

or, equivalently,

If KP, then ¬S

The challenge then is that to believe that KP, one must therefore believe that ¬S. And on what grounds can we so believe? How could we ever know that we aren’t being deceived by an evil demon, or aren’t a brain-in-a-vat? Surely anything we could advance as evidence for us not being so could just as easily be part of the deception.

Answering the sceptic

There are a number of avenues philosophers have considered in response to this sort of argument. One might, a la Descartes, argue that some knowledge (e.g. the Cogito) is possible even under the most extreme skepticism–some beliefs one just can’t be decieved into holding. Examples of these might be “I am. I exist.”, “I am experiencing pain” or “I have beliefs”.

But this response will not help the fallibilist–much of our everyday life depends on believing, and therefore claiming to know, things that we might be wrong about. So, perhaps S might not make infallible knowledge impossible, but what if amend KP to “fallible knowledge is possible”?

Another avenue, is to argue that we can know ¬S. Arguments for this go along a couple of lines. One can follow Moore and turn the whole argument on its head: argue that we have better reason to think that KP, via the deliverances of common sense, than to think that S, and so reject S because of our belief that KP. But this seems rather unsatisfying. Or one might follow Wittgenstein or Putnam and argue that there is a subtle contradiction in S. We will not explore this avenue here.

Interlude: Evidentialism and Reliabilism

As a third avenue, let us take a brief detour and give an account of what knowledge is. Above, we noted that no evidence seems like it could distinguish between S and ¬S. This suggests an account of knowledge called Evidentialism, viz.

One is warranted in believing that p if and only if one posseses evidence that favours p over ¬p

This is called an internalist account of knowledge, because whether one is warranted in believing p turns on factors internal to oneself, on what evidences you possess. But there is another type of account of what knowledge is, called externalism. In this, whether one is warranted in one’s beliefs turns on factors about the world external to oneself. A standard version of this is called Reliabilism, which broadly states:

One is warranted in believing that p if and only if the belief-forming process that caused one’s belief that p is in fact generally reliable (and was operating correctly, etc.)

Notably, on this view one can know that p in virtue of believing it on the basis of a reliable belief-forming process, even if one does not know or does not even believe that the process is reliable (though if one believes or knows that the process is unreliable that is a stickier matter).

Externalism to the rescue

So, let us suppose that some form of externalism is true. Then, if we are in fact not in a sceptical scenario there will in fact be the correct external conditions for us to have knowledge: if I am a real human with generally reliable eyes, and not a brain-in-a-vat, then the deliverances of my perceptions will be knowledge. Ergo we have the converse of (SP):

(EXT) If ¬S, then KP

So, this means I can coherently believe that ¬S, and on the basis of that believe that KP. Furthermore, for many sceptical scenarions such as being a brain-in-a-vat, if ¬S then I have reliable faculties the deliverances of which transparently entail that I am not a brain-in-a-vat–so I am warranted in believing I am not a brain-in-a-vat. So I have an entirely coherent web of beliefs available to me here, grounded in rejecting S, and therefore believing KP on the basis of believing that I do in fact have generally reliable faculties. And perhaps if coherentism is true, this is the best we could ever really ask for as an account of why we believe KP: a coherent web of beliefs of which KP is one.

The reader may object here though: have I not taken a bit of a leap here? The sceptic challenges me to account for why I reject S; have I not just answered this by saying “well, that’s just what I do”?

I think we can say a bit more on this. Recall from the above discussion that

KP if and only if ¬S

Which is to say we have a binary choice here: we can believe S or we can believe KP. Furthermore, we have to make this choice: there is no being agnostic about a matter as central as this. But is this really a binary choice? Can we actually choose either option? Here we finally recall the argument right at the beginning of this article. Every belief presupposes believing KP. Ergo, to believe S is self-undermining. If one believes S, then one believes KP and so believes ¬S.

This is the crucial distinction: one may not be able to demonstrate that ¬S, it may well be possible that S is true, but one cannot believe that S. This is reminiscent of Moore’s Paradox: it can very easily be true that “it is raining and I do not believe it is raining”, but it is impossible for me to assert or believe that sentence. Furthermore, we can also borrow Moore’s insight here: not only is it impossible to believe ¬KP, but we feel strongly compelled to believe KP by the deliverances of common sense, and we intuitively distrust the clever philosopher’s attempts to undermine this belief.

So the dilemma that the sceptic poses, of accepting or rejecting S, has the following features:

• It is a true dilemma.
• We are forced to take one of the horns.
• It is impossible for us to take the former horn.
• It is coherent and common sense for us to take the latter horn–and even if we do so with no claim to evidential justification this does no harm to our claims of everyday knowledge.

When you put it like that, is it really much of a challenge to account for the atheist’s choice?