Scientism seems to me to suffer from two fatal flaws. One is logical self-contradiction. For scientism is not a scientific claim about the universe but a philosophical claim, a claim in epistemology, about human knowledge, about the relation between human knowledge and science (or the scientific method). The claim is that only scientific knowledge, knowledge by the scientific method is reliable. But that claim cannot be verified or falsified by the scientific method. There is no empirical data, no mathematical measurement, and no logical reduction of the opposite claim (that knowledge extends beyond science) to a self-contradiction. If all knowledge claims that cannot in principle be verified or falsified by the scientific method are unreliable and should be rejected, then that very statement is unreliable and should be rejected.
My other objection is even simpler. How can the reliability of one kind of knowledge (science) disprove the reliability other kinds of knowledge? That’s like the ear telling the eye that it is unreliable just because it can’t hear anything, or like the mystic telling the scientist that he cannot have any valid knowledge of reality because he has never had a mystical experience.
--Peter Kreeft, Letters to an Atheist, 2014