I dislike categorising people into only two groups, but the conclusions I will draw in this article I find to be independent of this particular distinction, hence let us consider the assuredly religious and the decidedly non-religious. Of the many traits that separate the two groups, one which appears to be crucial, and which arises indirectly in many debates, is that the two groups consider different classes of information to be worthy of the name evidence. A devoutly religious person would consider personal experience to count as evidence, whereas a staunchly non-religious person would reject it and probably favour more tested information: that brought forward by science.
Why should this particular point be of interest, and thus why is it a contentious topic? When someone is questioned as to how they know whether any god does or does not exist, this someone may state some information in response which forms the basis of their faith. The questioner may reject such information, declaring it invalid. This is what is done with potential evidence by any rational sort; it is collected together, reviewed, and then conclusions are made based on its relevance and reliability.
Think of a scientist who measures the distance to a far off galaxy using only one method and one set of never-before-used equipment. Is there any way that this scientist could be wrong in his or her conclusion? Yes. It might be that the practical method the scientist used is flawed, and halves the true value. Perhaps the equipment is faulty, and instead doubles the correct value. Will the scientist return to the laboratory the next day and find the measurement is different, in which case, which measurement is incorrect, the previous one or the current one, or both? By how much was the measurement wrong, and what makes one measurement more reliable than another?
In science, diverse strength in numbers is the way to defeat these questions. Statistics offers a way around such inconveniences of the physical world. Many independent scientists will use different methods and different equipment to confirm or contradict a particular result. The principle is to vary all factors except those which are thought to influence the object or property under examination. Once nobody can think of any way in which the evidence is wrong, it would be considered correct.
The essence of good evidence is reliability. Reliability is simply how trustworthy the evidence is with regard to what you conclude from it, and what you assumed to get it. Does your evidence genuinely point to your deductions, or does it suggest something simpler? Is there another explanation of it? Reliable evidence should withstand every possible reasonable criticism of it. In order to achieve this, every other explanation for the evidence must be discarded, and conclusions drawn from the evidence must be minimised.
Quite simply, we examine the evidence and ask whether it can be wrong. If it can be, it’s not such good evidence.
One of the types of information that a religious person will often bring to the table is personal experience. Personal experience is a collection of information, unique to individuals, that has been obtained from one of the many sensory perceptions or mechanisms within the brain. Consider a religious person who decides their faith on personal experience. Is there any way that the religious person could be wrong in his or her conclusion? Yes. Did the person’s brain add in parts of an experience that weren’t there? Did the experience involve a sensation that was entirely psychological, such as pain? Did the person see or hear what they wanted to? Is this person biased towards a particular interpretation of their senses? How well does the brain record information in its surroundings? Could the person be lying? There is inherently a factor of unreliability within the brain.
These questions would be effortless to vanquish, except no other methods used to obtain the same information work. Specific personal experiences cannot be consistently replicated in other human beings. When non-human methods are used to try to identify the reason for an experience, absolutely nothing is found.
Personal experiences could technically be called evidence, seeing as they are simply artefacts of information, but they would be horrifically bad evidence, because they are thoroughly unreliable and almost always irreplicable. Because they’re so unreliable personal experiences will be systematically dismissed; they are never useful. There are too many unknown or indeterminable factors that influence the process; the method is not sound.