Here are the reasons why I converted, even though, it took quite a while for the full transformation to take place.
1: The consistent replacement of supernatural explanations of the world with natural ones.
When you look at the history of what we know about the world, you see a noticeable pattern. Natural explanations of things have been replacing supernatural explanations of them. Where thunder and lightning come from. Why people get sick. Why people look like their parents. How the complexity of life came into being.
All these things were once explained by religion. But as we understood the world better and learned to observe it more carefully, the explanations based on religion were replaced by ones based on physical cause and effect. Consistently. Thoroughly. Like a steamroller. The number of times that a supernatural explanation of a phenomenon has been replaced by a natural explanation? Thousands. The number of times humankind has said, "We used to think (X) was caused by physical cause and effect, but now we understand that it's caused by God, or spirits, or demons, or the soul"? Exactly zero.
Given this pattern, it's clear the chances of this are essentially zero. So close to zero that they might as well be zero. And the hypothesis of the supernatural is therefore a hypothesis we can discard. It is a hypothesis we came up with when we didn't understand the world as well as we do now... but on more careful examination has never once been shown to be correct.
2: The inconsistency of world religions.
If God was real and people were really perceiving him, why do these perceptions differ so wildly?
When different people look at, say, a tree, we more or less agree about what we're looking at: what size it is, what shape, whether it currently has leaves or not and what color those leaves are, etc. We may have disagreements regarding the tree - what other plants it's most closely related to, where it stands in the evolutionary scheme, should it be cut down to make way for a new sports stadium etc. But unless one of us is hallucinating or deranged or literally unable to see, we can all agree on the tree's basic existence and the basic facts about it.
This is blatantly not the case for God. Even among people who do believe in God, there is no agreement about what God is, what God does, what God wants from us, how he acts or doesn't act on the world, whether he's a he, whether there's one or more of him, whether he's a personal being or a diffuse metaphysical substance. And this is among smart, thoughtful people. What's more, many smart, thoughtful people don't even think God exists.
The explanation is God does not exist. We disagree so radically over what he is because we aren't perceiving anything that's real. We're "perceiving" something we made up; something we were taught to believe; something that the part of our brain that's wired to see pattern and intention, even when none exists, is inclined to see and believe.
3: The weakness of religious arguments, explanations and apologetics.
I have seen a lot of arguments for the existence of God. And they all boil down to one or more of the following: The argument from authority. (Example: "God exists because the Bible says God exists.") The argument from personal experience. (Example: "God exists because I feel in my heart that God exists.") The argument that religion shouldn't have to logically defend its claims. (Example: "God is an entity that cannot be proven by reason or evidence.") Or the redefining of God into an abstract principle... so abstract that it can't be argued against, but also so abstract that it scarcely deserves the name God. (Example: "God is love.")
And all these arguments are ridiculously weak.
Sacred books and authorities can be mistaken. I have yet to see a sacred book that doesn't have any mistakes. And the feelings in people's hearts can definitely be mistaken. They are mistaken, demonstrably so, much of the time. Instinct and intuition play an important part in human understanding and experience... but they should never be treated as the final word on a subject.
4: The increasing diminishment of God.
This is closely related to #1 (the consistent replacement of supernatural explanations of the world with natural ones). But it's different enough to deserve its own section.
When you look at the history of religion, you see that the perceived power of God has been diminishing. As our understanding of the physical world has increased - and as our ability to test theories and claims has improved - the domain of God's miracles and interventions, or other supposed supernatural phenomena, has consistently shrunk.
Examples: We stopped needing God to explain floods... but we still needed him to explain sickness and health. Then we didn't need him to explain sickness and health... but we still needed him to explain consciousness. Now we're beginning to get a grip on consciousness, so we'll soon need God to explain... what?
This is what atheists call the "god of the gaps." Whatever gap there is in our understanding of the world, that's what God is supposedly responsible for. Wherever the empty spaces are in our coloring book, that's what gets filled in with the blue crayon called God.
5: The fact that religion runs in families.
The single strongest factor in determining what religion a person is? It's what religion they were brought up with. By far. Very few people carefully examine all the available religious beliefs - or even some of those beliefs - and select the one they think most accurately describes the world. Overwhelmingly, people believe whatever religion they were taught as children.
Now, we don't do this with, for instance, science. We don't hold on to the Steady State theory of the Universe, or geocentrism, or the four bodily humours theory of illness, simply because it's what we were taught as children. We believe whatever scientific understanding is best supported by the best available evidence at the time. And if the evidence changes, our understanding changes. (Unless, of course, it's a scientific understanding that our religion teaches is wrong...)
Which leads me to the conclusion that religion is not a perception of a real entity. If it were, people wouldn't just believe whatever religion they were taught as children, simply because it was what they were taught as children. The fact that religion runs so firmly in families strongly suggests that it is not a perception of a real phenomenon. It is a dogma, supported and perpetuated by tradition and social pressure - and in many cases, by fear and intimidation. Not by reality.
6: The physical causes of everything we think of as the soul.
The sciences of neurology and neuropsychology are in their infancy. But they are advancing by astonishing leaps and bounds, even as we speak. And what they are finding - consistently, thoroughly, across the board - is that, whatever consciousness is, it is inextricably linked to the brain.
Everything we think of as the soul - consciousness, identity, character, free will - all of that is powerfully affected by physical changes to the brain and body. Changes in the brain result in changes in consciousness... sometimes so drastically, they make a personality unrecognizable. Changes in consciousness can be seen, with magnetic resonance imagery, as changes in the brain. Illness, injury, drugs and medicines, sleep deprivation, etc.... all of these can make changes to the supposed "soul," both subtle and dramatic. And death, of course, is a physical change that renders a person's personality and character, not only unrecognizable, but non-existent.
So the obvious conclusion is that consciousness and identity, character and free will, are products of the brain and the body. They're biological processes, governed by laws of physical cause and effect. With any other phenomenon, if we can show that physical forces and actions produce observable effects, we think of that as a physical phenomenon. Why should the "soul" be any different?
What's more, the evidence supporting this conclusion comes from rigorously-gathered, carefully-tested, thoroughly cross-checked, double-blinded, placebo- controlled, replicated, peer-reviewed research. The evidence has been gathered and continues to be gathered, using the gold standard of scientific evidence: methods specifically designed to filter out biases and cognitive errors as much as humanly possible. And it's not just a little research. It's an enormous mountain of research... a mountain that's growing more mountainous every day.
The hypothesis of the soul, on the other hand, has not once in all of human history been supported by good, solid scientific evidence. That's pretty surprising when you think about it. For decades and indeed centuries, most scientists had some sort of religious beliefs and most of them believed in the soul. So a great deal of early science was dedicated to proving the soul's existence and discovering and exploring its nature. It wasn't until after decades upon decades of fruitless research in this area that scientists finally gave it up as a bad job and concluded, almost unanimously, that the reason they hadn't found a soul was that there was no such thing.
7: The complete failure of any sort of supernatural phenomenon to stand up to rigorous testing.
Not all religious and spiritual beliefs make testable claims. But some of them do. And in the face of actual testing, every one of those claims falls apart like Kleenex in a hurricane.
Whether it's the power of prayer, or faith healing, or astrology, or life after death: the same pattern is seen. Whenever religious and supernatural beliefs have made testable claims and those claims have been tested - not half-assedly tested, but really tested, using careful, rigorous, double-blind, placebo-controlled, replicated, etc. etc. etc. testing methods - the claims have consistently fallen apart. Occasionally a scientific study has appeared that claimed to support something supernatural... but more thorough studies have always refuted them. Every time.
Supernatural claims only hold up under careless, casual examination. They are supported by wishful thinking and confirmation bias and our poor understanding and instincts when it comes to probability and our tendency to see pattern and intention even when none exists and a dozen other forms of cognitive bias and weird human brain wiring.
8: The slipperiness of religious and spiritual beliefs.
Not all religious and spiritual beliefs make testable claims. Many of them have a more "saved if we do, saved if we don't" quality. If things go the believer's way, it's a sign of God's grace and intervention; if they don't, then God moves in mysterious ways and maybe he has a lesson to teach that we don't understand and it's not up to us to question his will. No matter what happens, it can be twisted to prove that the belief is right.
That is a sure sign of a bad argument.
Here's the thing. It is a well-established principle in the philosophy of science that, if a theory can be supported no matter what possible evidence comes down the pike, it is useless. It has no power to explain what's already happened, or to predict what will happen in the future. The theory of gravity, for instance, could be disproven by things suddenly falling up; the theory of evolution could be disproven by finding rabbits in the pre-Cambrian fossil layer. These theories predict that those things won't happen; if they do, the theories go ****. But if your theory of God's existence holds up no matter what happens - whether your friend with cancer gets better or dies, whether natural disasters strike big sinful cities or small God-fearing towns - then it's a useless theory, with no power to predict or explain anything.
What's more, when atheists challenge theists on their beliefs, the theists' arguments shift and slip around in an annoying "moving the goalposts" way.
9: The failure of religion to improve or clarify over time.
Over the years and decades and centuries, our understanding of the physical world has grown and clarified by a ridiculous amount. We understand things about the Universe that we couldn't have imagined a thousand years ago, or a hundred, or even ten. Things that make your mouth gape with astonishment just to think about.
And the reason for this is that we came up with an incredibly good method for sorting out good ideas from bad ones. We came up with the scientific method, a self-correcting method for understanding the physical world: a method which - over time and with the many fits and starts that accompany any human endeavor - has done an astonishingly good job of helping us perceive and understand the world, predict it and shape it, in ways we couldn't have imagined in decades and centuries past. And the scientific method itself is self-correcting. Not only has our understanding of the natural world improved dramatically: our method for understanding it is improving as well.
Our understanding of the supernatural world? Not so much.
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