It isn’t easy to determine what claims should be believed and what claims shouldn’t be. There is no easy answer to the question “when should we believe”. However, this is a very important discussion to have. It may not always be obvious whether we should believe something or not, but it is possible to determine whether or not a belief is rational.
But, before I discuss how we can determine whether a belief is rational, I should discuss when it is necessary to determine if a claim should be believed. This is a difficult concept for a lot of people. Many people want to be overly skeptical, and others aren’t skeptical enough. So when should we determine if our beliefs are rational? When our beliefs have a significant impact on us or on those around us. But what does that mean? It means that everyday claims don’t tend to count. If I were to say “I want cereal for breakfast,” Withteeth does not have to be skeptical of that claim. He doesn’t have to demand that I prove that I want cereal. If I were to say that I have class at 4:30, Withteeth still does not have to be skeptical. However, if I were a drug addict who had a habit of using the “I have class” excuse to sneak out and get high, then he has reason to be skeptical. He can demand that I show him my class schedule, and he can even follow me to class, watch me enter the classroom, and ensure that I don’t leave the entire time. In short, he can know for sure that I went to class, and he can base his belief on that knowledge. But he shouldn’t believe that I have class at 4:30 without at least looking at my schedule if I am known to be untrustworthy. But, while this example has a significant impact on our lives, it doesn’t effect anyone else. There are other things that have a lot more impact and, as such, require a lot more evidence. Belief in God is one example of such a claim. If God exists, then his existence, assuming the Bible is an accurate representation of God’s personality, has a significant impact on the world as a whole. As such, existence of God requires more evidence than whether or not I am skipping class. Showing that God exists is a start. This can be done in a number of ways. The easiest would be to point to God and say “there he is” and for God to then do something that proves he is God. Without that, though, you can also prove God exists by showing that something could not exist without God. This is the route that a lot of theologians take. The problem is that it is difficult to prove that something could not exist without God. But, even if God were proven, that is just a start. You would still have to prove that this God, again assuming the Bible is accurate, is the God of the Bible and not some other God. As you can see, this is a tall order.
So how do we know that our beliefs are rational? By determining whether they are probable. Absolute knowledge isn’t necessary to say that a belief is justified (it isn’t even necessary to say that you know something). But there is a degree to which you can say that a belief is rational. Evidence is how we determine the probability of a belief being true. Going back to the skipping class example, how would Withteeth know that it is rational to doubt my claim that I have class at 4:30? If he has caught me skipping class a number of times before, then it is likely that I would do again. If it is the middle of the semester and I have never gone to a 4:30 class before, then it is likely that I am lying. If he has heard from a number of other people that I constantly skip that class, then he has reason to believe that I am lying. He is not rational in believing that I will skip my class, or that I don’t have class, if I have only skipped one class that he knows of. He is not rational in believing that I don’t have a class if I am always at school from 9am to 6pm and he doesn’t know when each class is. And he is not rational in believing that I’m lying if one person told him that I skip the class a lot. He doesn’t have enough evidence to make that claim. So how would he determine the probability that I will skip class? He would need to do some research. He could go to my professors and ask them, but they may or may not know. He’d also have to go to my classmates. Assuming that a number of them know who I am and remember my relative class attendance, he could create an average of how often I probably attend class. From there he can create a probability of my likelihood of my skipping class. Though his evidence would be much stronger if he had a more concrete evidence. For example, if I had a camera watching me while I sat in class during every class I attended, then he could say for a fact how many classes I skipped. Then his probability would be more accurate.
But how high does the probability have to be for the belief to be justified? That depends on the claim. If the probability is 51%, that should be enough for the claim “I have class at 4:30.” But a claim like “God exists” requires a higher probability.
So when is it rational to believe a claim? That depends on the claim. It is always rational to believe the claim “I want cereal for breakfast,” but, depending on certain characteristics related to the person making the claim, it might be irrational to believe the claim “I have class at 4:30.” However, in most cases. It is perfectly rational to believe that claim as well. But it is less rational to believe the claim “God exists” because of the lack of evidence and the low probability. I’ll link to a few sites that may help with further understanding of this concept.