Scientism: "Unlike the use of the scientific method as only one mode of reaching knowledge, scientism claims that science alone can render truth about the world and reality. Scientism's single-minded adherence to only the empirical, or testable, makes it a strictly scientific worldview, in much the same way that a Protestant fundamentalism that rejects science can be seen as a strictly religious worldview. Scientism sees it necessary to do away with most, if not all, metaphysical, philosophical, and religious claims, as the truths they proclaim cannot be apprehended by the scientific method. In essence, scientism sees science as the absolute and only justifiable access to the truth." (1)
Scientism, in short, rejects the possibility of forms or methods of knowledge other than science. "Certainly, it requires the almost complete abandonment of any metaphysical or religious discussion, (and arguably also any ethical discussion), on the grounds that these cannot be apprehended by the scientific method, which is very limiting for a supposedly all-encompassing doctrine." (2)
One of the problems with scientism is that "It has been argued that Scientism, in the strong sense, is self-annihilating in that it takes the view that only scientific claims are meaningful, which is not itself a scientific claim. Thus, Scientism is either false or meaningless." (3)
Scientism illustrates two important and general concepts that feature in a discussion of science and religion: the problem of self-referential statements, and meta-statements.
A simple example of a self-referential statement is "This statement is false." If the statement is taken as true, then it means that the statement is false. If the statement is false, then it is false that the statement is false, which means that the statement is true. So either way, there is a contradiction.
A more involved example is due to Bertrand Russell, and is known as the "barber paradox". There is an island that is inhabited by men, and only men. On that island there is one and only one barber. Of all of the men on the island, the barber and only the barber is allowed to shave another man. There are two and only two classes of men: men who shave themselves, and men who do not shave themselves. Men who do not shave themselves are shaved by the barber, and men who do shave themselves are not shaved by the barber. In a less rigorous sense, a man either shaves himself or is shaved by the barber, but not both.
So, the question is: who shaves the barber. If the barber shaves himself, he is in the category of men who are not shaved by the barber, hence he does not shave himself. If the barber does not shave himself, he is shaved by the barber, which means that he shave himself. Thus the barber cannot shave himself, nor can he be shaved by someone else, but he is clean shaven, which is impossible from within the system. More on that in a minute.
Given a system of statements, a meta-statement is a statement about that system, and is not a part of the system itself. The distinction is important, because mistaking a meta-statement as part of the system leads to all sort of confusion and flawed reasoning. For example, the statement that evolution is without purpose or meaning is not a scientific statement within the theory of evolution - it is a statement about evolution. Whether evolution has meaning and purpose is a theological or philosophical question, and cannot be answered by science. On the other hand, the mechanism of evolution is a scientific question, which can be answered by science, but not by theology or philosophy.
Returning to the barber paradox, it can be resolved only if there is someone who is not one of the men on the island, that is, someone who is not part of the system of men on the island. Because he is not part of the system, he is not bound by the rules (not restricted by the rule that only the barber can shave someone else), so he can shave the barber. The paradox is an example of a theorem, known as the Incompleteness Theorem, proven by the Austrian logician Kurt Godel in the early 1900's, which says that if a system is complete, then it is not consistent; and if a system is consistent, then it is not complete. (Technically speaking, Godel's proof applies to logical systems at least as complex as ordinary arithmetic. Whether the theorem can be generalized to less formal systems is a question of some dispute. I personally think that it can.) Because the system of men is complete (the statements defining the system exhaust all the possibilities), it then follows from the Incompleteness Theorem the system is not consistent, as shown by the fact that it requires that the barber both shave himself and not shave himself.
The point that I'm trying to make here is that there are two classes of statements with respect to science: scientific statements, which are statements within science; and meta-scientific statements, which are statements about science but are not part of science themselves. Using the example of evolution, a statement that evolution does or does not have a purpose is a meta-scientific statement. That doesn't mean that the question of purpose is not important, nor does it mean that is it not discussable. It does mean that the question belongs to the realm of theology or philosophy. In other words, it has to be evaluated by non-scientific disciplines. The problem with scientism is that it asserts that such disciplines are invalid because only science can provide truth, and those areas cannot be addressed by science.
In conclusion, it is important to keep in mind, when discussing science and religion, which statements are scientific, and which are meta-science.