Let’s kick things off with an old philosophical argument, first put forward by Descartes in ‘Meditationes de prima philosophia’, and now updated into a more modern form. Here it is:
The Brain in a Vat (BIV) Argument:
(1) It is possible that I am in fact a BIV, kept there by a mad scientist who systematically deceives me about the truth of most of what I believe, feeding me my perceptions of the external world through cables attached to my brain.
(2) It is impossible for me to definitely rule out the possibility that I am a BIV.
(3) If I cannot know that I am not a BIV, then my beliefs about an external, non-mental reality cannot constitute knowledge.
Conclusion: My beliefs about an external, non-mental reality do not constitute knowledge.
In other words: logically we have no proof that there is any external reality whatsoever – the only thing that we can ever know for sure is that we personally exist (Cogito ergo sum and all that). Now let’s add in another argument, the problem of induction, which states that there is no logical reason for supposing the past is a good guide to the future.
You would think that conclusive proof that no one can be certain there is an external world out there, and that even if there is one we can’t in fact draw any conclusions on it based on previous experience, would present a problem. To the best of my knowledge however, despite the logical impeccability of these arguments, no one actually lives in a manner consistent with them. People do not sit on chairs as gibbering wrecks, afraid to move in case the world spontaneously explodes – neither do people confidently walk out in front of buses because they cannot possibly have any knowledge that doing so will be harmful.
So, how do most of us get through the day? Well, most of us simply ignore these problems: we implicitly trust the evidence of our senses, and in a nice piece of circular reasoning, figure that if reasoning based on past experience has always worked in our past experience, it will work in the future. We will continue to act in a manner consistent with, and make predictions about, a genuine external reality, even after it is shown to us that there is no possible good reason to do so. The answer to getting through the day seems to be faith - a belief, not resting on logical proof or material evidence, that not only is there a) a genuine external world present outside of our minds, but b) our past experience is a good guide to the future. I’ll repeat – these are beliefs that we have no good reason to believe, and yet everyone acts as if they do believe them.
So we all rely on faith to some extent, but by its very nature it is a difficult thing to judge. How much faith should we have? We’ve made the step to believing there is an external world out there, having no reason to do so. Why then draw the line there? What about fairies? Or teapots in orbit outside of Mars? Or in fact anything anyone may choose to mention? After all, the cognitive step is the same. There is no more evidence for the first statement than any of the others. This is a problem if we want to have any hope at all of discriminating true knowledge from false.
For my part, the answer lies in the principle of parsimony – "Entities should not be multiplied beyond necessity." If we postulate the minimum necessary principle to make contact with the outside world – the principle that it does indeed exist - and that in addition we can use the past as a good guide to the future, then all other possible claims can be investigated using only these two unsupported beliefs. The scientific method can then be applied, and a reasoned conclusion reached on any other statements we may care to make regarding the external world, say that fairies are real, or that objects accelerate with F=ma. Thus through parsimony there is the luxury of allowing oneself an outside world to relate to and make predictive claims about, whilst retaining the right to examine any further claims with a sceptical eye, keeping, as the saying goes, “an open mind, but not so open that your brains fall out.”
Which is nice.