I want to first start off by saying that I’m not one of those atheists that blames all of the world’s problems on religion. Religious belief has certainly been the root cause of some of humankind’s problems throughout our history, but it is no way is the root cause of all of them. There are many reasons why we harm one another and our environment that has little to no religious motivation. So when I criticize the social effects here of religious belief, I am by no means claiming that religion is the root of all evil.
With that out of the way I want to articulate as best I can why I think the atheistic or naturalistic worldview is perfectly rational and justified and is preferable to theism. The atheistic worldview is built on the naturalistic worldview, also known as metaphysical naturalism. For short here, whenever I refer to naturalism or the naturalistic worldview, I will be referring to metaphysical naturalism. Metaphysical naturalism is roughly defined as “a worldview with a philosophical aspect which holds that there is nothing but natural elements, principles, and relations of the kind studied by the natural sciences”, or “the thesis that nothing besides the natural world, or nature, exists”. There is also methodological naturalism, which is an aspect of naturalism that assumes metaphysical naturalism to be true when conducting the scientific method (meaning to assume natural causes when doing scientific research since supernatural causes are out of the reach of the scientific method).
Now all of science operates more or less under both aspects of naturalism. Even though this is true, science doesn’t have to be committed to the idea that the natural world is all that there is, and a quick check of history will tell you that many of the famous scientists (or natural philosophers as they were once called) of years past did assume supernatural causality was responsible for observed phenomenon. When Isaac Newton for example was at the limits of his knowledge in understanding the complex rotations of the planets under his theory of gravity, he appealed to the supernatural to explain that which he could not. It took another genius, Albert Einstein, to discover what gravity was by means of a natural explanation – general relativity. Over the years as our scientific knowledge grew larger, these supernatural assumptions were slowly debunked and replaced with natural explanations. And to date, everything that science can currently explain is explained by naturally occurring phenomenon. This has resulted in the widespread prevalence of naturalistic causality and explanation becoming the preferred worldview and methodology of choice for scientists. So if it is true as many theists claim that knowledge of the supernatural dimension is forever off limits to the scientific method, then scientists are justified in their metaphysical and methodological naturalism, because such unreachable knowledge is never objectively observed, fails to sufficiently explain anything, and therefore might as well not even exist.
To claim that a being or phenomenon exists outside of what science can determine through observation and experimentation, is to open up the imagination to potentially unlimited amounts of conjecture as to what might exist beyond our senses. You could literally posit the existence of anything that the human mind could conjure up, customize it anthropomorphically to your liking, and say that it’s off limits to being empirically verified. No one will have any way to know for sure if this being or phenomena exists or not, but if its alleged effects could be explained naturalistically or shown to violate known physical laws, then the naturalist is justified in at least disbelieving this being or phenomenon exists until adequate evidence is produced.
Although, I strongly believe the natural world is all that exists, I don’t claim to know this empirically. It is impossible as far as I know to prove a negative (i.e. that god doesn’t exist). All the atheist or naturalist can hope for is that plausible, natural alternatives can be produced to explain the existence of things believed to require supernatural causes. One criticism of naturalism is that it cannot be scientifically proven. Although that is true, it also cannot be proven that we’re not all living inside a giant computer simulation and that the reality we experience is not in fact real at all. No one can empirically prove or disprove such a claim, and anyone who doubts such a claim, more or less has to take it for granted that their cognitive faculties are reliable. Naturalism, much like atheism, cannot as far as I know be empirically proven, but this is not at all required. All the naturalist/atheist merely has to demonstrate is that there is no valid evidence for the claims made by theists that the supernatural exists and that there exists natural explanatory alternatives, and he or she is justified in holding the disbelief in the supernatural.
When it comes to the claim made by some naturalists that we should only believe what can be scientifically proven, an idea known as scientism, I partly disagree here. First, anything that can be scientifically proven we know to be true, unless all of our cognitive faculties are unreliable – which we have no evidence for and no strong reason to believe. Second, the existence of truths that cannot be scientifically verified, like mathematical and logical truths, aesthetic truths, metaphysical truths (like believing we are not living in a computer simulated reality), and ethical truths are only to a certain degree not scientifically provable. We cannot scientifically prove that 1 + 1 = 2, we cannot use science to prove logic, and we cannot even use logic to prove logic. We could show that if our universe behaves logically by fully understanding its laws of physics, then it would make sense why mathematical and logical truths exist, but ultimately these kinds of truths might have to be accepted as a given set of axioms. Science can show us why we might prefer certain kinds of beauty from the socio-cultural and biological evolutionary process, but science cannot prove whether a specific painting or work of art is beautiful. Aesthetic beauty fundamentally lies in the eye of the beholder. Ethical truths cannot be determined alone by science because once you interpret the scientific data that a given set of ethical values hinges on, you will have to make sense of them using philosophy. Although science does indeed play a role in determining moral values, it doesn’t have the final and only word on morality.
In short, just because we cannot empirically prove that the natural world is all that exists, the naturalist/atheist is rationally justified in adopting naturalism because there is no evidence to the contrary. When it comes to the existence of extraordinary claims, like the supernatural, I essentially employ a verificationist attitude: when adequate evidence is produced, I will incorporate it into my belief system, but until then, the default position is disbelief. This is why atheists are called skeptics. We believe a healthy dose of skepticism is needed in our lives to separate fact from nonsense. This is because all kinds of people are making fantastic claims not only about the supernatural, but also about the paranormal, and they’re offering little to no evidence to back up these claims. As a skeptic, I just can’t go on believing that such claims are all true without adequate evidence because that’s being gullible; and being an agnostic on all such claims would then force me to consider the truthfulness of some of the most imbecile and irrational ideas mustered out of every half-thinking brain. Rather, if the assertion is not knowable a priori, or backed up with adequate evidence, the default position should be disbelief – especially if it violates all the known laws of physics. Therefore, since no such evidence exists that supernatural occurrences and agencies are real, the naturalist is perfectly justified in disbelieving in every unscientific claim.