Karl Popper was concerned with answering the question S: “What is science and what distinguishes it from pseudoscience?” In his "Science: Conjectures and Refutations", Popper states the problem as: “Is there a criterion for the scientific character or status of a theory?” More generally, one can ask what is sui generis (or special) about science or its methodology. He was not concerned with whether or not a scientific theory was true or false; rather, he was concerned with what properties made a given theory properly scientific. False scientific theories are still scientific. Answering S is known as the Demarcation Problem. While most present philosophers would agree that Popper's answer is naive, I think it does capture some good intuitions.
Popper’s answer to S is that scientific hypotheses (or statements) need to be
falsifiable. He reaches that conclusion by comparing Adlerian and Freudian psychological theories on the one hand and General Relativity on the other.
I found that those of my friends who were admirers of Marx, Freud, and Adler, were impressed by a number of points common to these theories, and especially by their apparent explanatory power. These theories appeared to be able to explain practically everything that happened within the fields to which they referred. The study of any of them seemed to have the effect of an intellectual conversion or revelation, opening your eyes to a new truth hidden from those not yet initiated. Once your eyes were thus opened you saw confirming instances everywhere: the world was full of verifications of the theory. Whatever happened always confirmed it. Thus its truth appeared manifest; and unbelievers were clearly people who did not want to see the manifest truth; who refused to see it, either because it was against their class interest, or because of their repressions which were still 'un-analysed' and crying aloud for treatment.The problem with Marx, Adler, and Freud (according to Popper) is that these theories do not provide conditions under which we would conclude that they are false. They are consistent with all possible situations. More importantly, Adlerian and Freudian psychoanalysis provide distinct (and sometimes contradictory) explanations for human behaviour. They cannot both be true, but they cannot be distinguished on the basis of experiment.
Furthermore, Popper writes:
I could not think of any human behavior which could not be interpreted in terms of either theory... It began to dawn on me that this apparent strength was in fact their weakness.That these theories could not be shown to be wrong was actually reason to reject them, according to Popper. If the theory cannot be falsified (even in principle) this is a reason to think that whatever explanatory power it has is vacuous.
Now compare Popper's idea with a famous quote from C.S. Lewis (from his "Is Theology Poetry?" and often quoted by theologian Alister McGrath):
I believe in Christianity as I believe that the sun has risen: not only because I see it, but because by it I see everything else.The quote seems to reflect exactly the same idea which Popper expresses. Except that for Lewis this kind of explanatory power is seen as justificatory. I think this gives an interesting insight into a sort of peculiarity about the justifications commonly given by Christians.
Amongst contemporary Christians, Christianity (or their "relationship with Jesus") is often understood as a transformative lens through which one views the world and by which one decides how to act (Popper: "The study of any of them seemed to have the effect of an intellectual conversion or revelation, opening your eyes to a new truth hidden from those not yet initiated"). All experience is interpreted through that lens; extraordinary and noumenus experiences are explained in terms of the Divine instead of in terms of psychology.
One can also consider the sort of theodicies that are often given in response to the Problem of Evil to see that such a view directly contradicts what Popper thought were good scientific virtues. To many theists, there is simply no such thing as a disproof of their beliefs (Popper: "Whatever happened always confirmed it").
I remember sitting in a Philosophy of Religion class and having the following sort of conversation (these quotes are fictional, but they accurately capture my memory of what was expressed):
Person 1: I think religion helps people live better lives. If you look at religious people, you see people who are better off than non-religious people.
Me: Actually, studies show that the Scandinavian countries rank the highest on all of the indices of personal prosperity -- life expectancy, health, happiness, educational attainment, etc. The Scandinavian countries are also the least religious countries in the world. By contrast, the most religious countries in the world rank the lowest on these indices.
Person 1: I think that actually supports what I'm saying. People turn to religion when they are living in horrible conditions because they have nothing else to turn to.
Interestingly, this might also help to explain the large number of theists who claim that atheists secretly believe in God or really just hate God (Popper: "unbelievers were clearly people who did not want to see the manifest truth"). Because theistic belief forms such a strong paradigm under which such believers operate, one might think that it becomes almost inconceivable for the believer that there exist those who have radically different ideas about the nature of reality. It can also explain the difficulty that some believers (who I have met) have in understanding what it's like to be a non-theist. How many times are we asked so you really don't believe in God? Or but how can you do/think about x, y, or z?