A few months back I encountered a guy at a bar, who asked me about a blasphemous-looking tattoo I have on my right arm. I told him that I used to be religious, but now I'm not. He asked me why. Trying to keep things as terse as possible since I wanted to get back to having fun with my friends instead of getting sucked into a debate about theology, I explained that I had been a devout Christian for many years, but after studying theology and logic I found too many things wrong with it to keep believing.
He replied, "Like what?"
Oh boy. I knew where this was headed. I gave a very quick example or two, and finished with, "I believe it when I see it." By which I really meant that I accept it when it can be empirically verified, but I was at a bar for crying out loud. While an evangelical Christian would jump at an opportunity to talk to a nonbeliever, I'm not out to convert people and just wanted to get back to my friends.
Anyway.... He replied with the old, "Believing is seeing" canard. I've actually heard that one quite a bit. There are a lot of believers who insist that by requiring supernatural claims about reality to be subject to the same standard of evidence as any other statement about reality, that I'm missing the point – that if I believed first, without evidence, then the evidence would become apparent to me as God did his amazing work through me.
Nonsense by any other name....
Fortunately, there's a name for this kind of nonsense, and it's called confirmation bias. It means that when you make an unsubstantiated assumption, it will affect your perception of reality and you will subconsciously affirm information that supports your assumption while subconsciously discarding that which does not.
A good model to use to illustrate the mechanism of this fallacy would be the Person-Centered Theory model pioneered by the psychotherapist Carl Rogers, which YouTube personality TheraminTrees used in his video "Atheism as Congruence". In this model, information is divided into two parts: Experience, which is the multitude of raw data we receive through our senses, and Self-Structure, which is the means by which we organize the data into meaningful, useful information. Congruence describes the way we attempt to understand and function in the world by matching our self-structure with our experiences.
In the case of a confirmation bias, no experience has informed the belief, which is held as part of self-structure. In fact, the "seeing is believing" fallacy explicitly requires that we form a belief independently of our experiences. Thus, in order to seek congruence between self-structure and experiences, information will be selectively retained or discarded in order to reinforce the belief.
Believing is selectively filtering information which reinforces your bias
Prayer serves as an excellent example of confirmation bias. Imagine a loved one of yours has cancer, and you pray to God that they will recover. Sure enough, they recover. Now, improbable recoveries happen often enough – take Lance Armstrong's incredibly unlikely recovery from stage 3 testicular cancer, or any of the roughly 5% of people who will survive five years after being diagnosed with pancreatic cancer. Indeed, the odds of surviving cancer is a statistically predictable phenomenon. "Beating the odds" means falling into that percentage of people who survive, no matter how unlikely. But to maintain congruence between your experiences and your self-structure, you would perceive your relative's survival as an answered prayer. Had your relative not survived, you would not discard the belief – after all, it's likely that you have found many other trivial events to selectively reinforce your belief, which have served to strongly integrate the belief with your self-structure. Rather, you would manipulate your interpretation of the events to allow your belief to thrive. You might say, "It was just God's will", or "I didn't have enough faith."
Most people do not acquire their religious or spiritual beliefs through a rigorous study of logic, evidence, and comparative religion, rationally deducing which faith-based claims to knowledge are the correct ones. The vast majority of people acquire religious beliefs through familial and sociocultural pressures. This means that a confirmation bias is often powerfully integrated into someone's self-structure very early on in their life. Take my own experience as a Christian: I was raised to believe in Jesus and follow the tenants of Christianity, but was never deeply involved in church. But as a teenager, when I was introduced to a fanatical, charismatic group of my fellow teens who were guided by similarly charismatic adults, I had no reason to question the bizarre behavior that I encountered – casting out demons, falling on the floor in convulsions, speaking in tongues, miracle healing, etc. My confirmation bias was already deeply integrated with my self-structure, so the social pressures presented simply encouraged me to selectively reinforce beliefs I already held, rather than treating them with skepticism.
Confirmation bias is not innocuous. It's one of the most fundamental components of religious belief, and it's one of the reasons why believers often seem impervious to reason. William Lane Craig, my favorite apologist punching bag, has gone so far as to state that even if he thought the evidence was against him, he would still believe in God because his belief is derived primarily from personal revelation – his encounter with the "holy spirit". It can lead to foolish and ridiculous behaviors as it did with me, as well as creating and deepening prejudices and contempt for those whose beliefs conflict with our own. To avoid the pitfalls of confirmation bias, evidence must always be the basis for our understanding of reality.