When people wish to exclude others from considering any possibility that might be associated with the paranormal or the religious, such people often use a type of reasoning I might call the “that's religion, not science” argument. Most commonly this is used as a rationale for completely excluding any thought that our universe may be the result of something more than just blind chance. If someone, for example, suggests that the origin of the universe may have been some kind of divine creation, such a hypothesis may be excluded under the grounds that such thinking is not science, but religion.
The same reasoning can be used to try to justify a kind of “gag rule” in scientific publications – a rule forbidding mention of anything that might be considered from the world of the paranormal or the religious. Got some evidence that suggests the possibility of some design or purposeful direction in the origin of life or the evolution of life? Sorry, discussing that is not allowed because that's religion, not science. Got some evidence based on near-death experiences that there might be such a thing as a soul that survives death? Sorry, you can't discuss that in a scientific publication, because that's religion not science. Got some evidence that there may be some power of the mind beyond that which neurology can account for? Sorry, you can't present that evidence because that's religion not science. You get the idea. This “that's religion not science” argument ends up being very convenient for the materialist or physicalist, and it has a superficial plausibility.
This type of reasoning involves a kind of “science and religion apartheid” thinking. The idea is that there's the science box and the religion box (like two cardboard boxes on different parts of the floor), and that we have to place a particular idea or writing in either one box or the other. Paraphrasing the famous statement about East and West, you might express this assumption by stating this slogan: science is science, and religion is religion, and never the twain shall meet.
But is this type of reasoning valid? No, it isn't, as I can show with an important historical example. In the 1920's a Belgian priest by the name of Georges Lemaître first proposed the idea that the universe suddenly began ages ago in a state of inconceivable density that he called a primordial atom. At the time the idea that the universe had a sudden origin was an idea from the world of religion, not the world of science. For one thing, there was no known evidence for such a theory. Scientists favored a different idea, that the universe had existed forever. No doubt many scorned Lemaître's idea, saying “That's religion, not science.” But in the next few decades the evidence for such a sudden beginning of the universe began piling up. Now scientists accept such an idea, which they call the Big Bang theory.
So we have here an important example of an idea from the world of religion – the idea of the sudden origin of the universe – that started out as unsubstantiated (not science), but then actually became science (and not just trivial science, but one of the most important findings of modern science). What lesson must be draw from this example? The lesson is: an idea should not at all be excluded from further consideration by scientists merely on a basis such as “that's religion, not science.” This example proves that an idea that originally seems more religious than scientific may end up becoming an important part of science.
So the “that's religion, not science” type of reasoning is not valid as some basis for exclusion. But why exactly is this reasoning fallacious? I can give two reasons.
Reason #1: There Are Several Definitions of Science
The “that's religion, not science” type of reasoning takes advantage of the fact that science is defined in two different ways that are quite different. The first definition is what we may call the “facts on the shelf” definition. People sometimes speak of science as the body of facts collected by scientists. Using this definition, you can attempt to categorize almost all theoretical ideas that scientists discuss as being “not science,” on the grounds that they are not yet proven facts. Just as you can exclude almost everything from the world of the paranormal or the religious as “not science,” you can exclude many scientific hot topics such as string theory, the cosmic inflation theory, the multiverse, neo-Darwinism, and many other theories which are not yet regarded as well-proven as, say, the existence of gravitation or the existence of bacteria.
But there's a second definition of science – what we can call the “process” definition. According to this definition, science is the process of seeking truth through systematic efforts that involve observations, experiments, and theorizing. According to this definition, almost anything that involves honest, systematic and well-organized observations, experiments, or evidence-based theorizing is science (whether it be professional science or what is called citizen science carried out by non-professionals). So according to this definition, almost everything that is typically excluded on the basis of being “religion not science” is actually science. That doesn't mean that it's proven to be entirely correct, but merely that it does fall under the category of “the process of seeking truth through systematic efforts that involve observations, experiments, and theorizing,” which is one of the main definitions of science.
So, in fact, it is not all clear that those items typically excluded as being “religion, not science” are in reality “not science.” They may well be science (either today or in the future) depending on which definition of science you use.
Reason #2: Ignoring the Possibility of an Overlap
The second major fallacy in “that's religion, not science” type of reasoning is the assumption that the realm of science truth claims and the realm of religious truth claims are mutually exclusive areas that can never overlap to any degree (so that once we have identified something as a religious truth claim it can be excluded as a science truth claim). The assumption may be illustrated by Model 1 in the diagram below.
Such an assumption may be incorrect, because there may be some overlap between the realm of religious truth claims and the realm of science truth claims. The truthful situation may be as illustrated as Model 2 in the diagram above, in which some truth claims can exist in both the realm of science and the realm of religion.
What reasons are there for thinking that the correct model is the second of these models, not the first? For one, there's the simple fact that human realms of thought do not naturally tend to be entirely mutually exclusive without any overlap. For example, there is a political realm of thought that includes the notion that “all men are created equal.” But the fact that a truth claim of human equality has been made in the realm of politics does not mean that we should exclude it from the realm of science. For example, if you were doing a scientific study on intellectual abilities in different races, it would hardly make sense to say, “I must at the beginning rule out the idea that all races are equal, because the idea that all men are equal belongs to politics, not science.”
Another reason for thinking that the second diagram is the correct one is that we have the huge historical example of the claim “the universe suddenly originated.” That is a currently a claim existing in both the realm of science and the realm of religion. So this proves there is some overlap between the realm of science truth claims and the realm of religious truth claims. We could easily add another “overlap” example involving claims that the universe is exquisitely fine-tuned. I could make a list of statements along these lines by scientists and religious people, without identifying the person who made the statement; and you would have a hard time distinguishing between the statements by the religious and the statements by the scientists. One might also add as examples of overlap between science and religion some of the items that Fritjof Capra called attention to in his famous book The Tao of Physics: An Exploration of the Parallels Between Modern Physics and Eastern Mysticism.
What is the proper attitude that a scientist should take to a truth claim that he has identified as one that is being made by some religion or religious person? The proper attitude is not one of automatic exclusion because of such a thing, but instead an attitude of indifference toward such a thing. For example, if you are a scientist considering whether there is some evidence for the soul, you should not be saying, “I will rule that out because some religious people believe it,” but instead you should be saying, “I will pay no attention to how many people believe it, but judge the matter purely on the facts and the evidence.”
But such a principle is ignored by many who try to keep science as illogically exclusionary as a 1950's Alabama country club.