Alfred Centauri refers us to Scientism: Why Science is a Bad Philosopher, an article worth reading. The definitions of 'scientism' the author approvingly quotes suggest that he would accept my characterization:
Scientism is a philosophical thesis that belongs to the sub-discipline of epistemology. It is not a thesis in science, but a thesis about science. The thesis in its strongest form is that the only genuine knowledge is scientific knowledge, the knowledge generated by the natural sciences of physics, chemistry, biology and their offshoots. The thesis in a weaker form allows some cognitive value to the social sciences, the humanities, and other subjects, but insists that scientific knowledge is vastly superior and authoritative and is as it were the 'gold standard' when it comes to knowledge. On either strong or weak scientism, there is no room for first philosophy, according to which philosophy is an autonomous discipline, independent of natural science, and authoritative in respect to it. So on scientism, natural science sets the standard in matters epistemic, and philosophy’s role is at best ancillary. Not a handmaiden to theology in this day and age; a handmaiden to science.
The author appreciates that scientism enjoys no scientific support. He appreciates that it is not a scientific thesis that can be verified or falsified by scientific procedures. But he fails to give a sufficiently powerful argument against it.
One problem with strong scientism is that it is self-vitiating, as the following argument demonstrates:
a. The philosophical thesis of strong scientism is not an item of scientific knowledge.
b. If all genuine knowledge is scientific knowledge, then the philosophical thesis of strong scientism is not an item of genuine knowledge.
Hence one cannot claim to know that strong scientism is true if it is true. For if it is true, then the only genuine knowledge is scientific knowledge. But strong scientism is not an item of scientific knowledge. So scientism, if true, is not knowable as true by the only methods of knowledge there are. How then could the proponent of strong scientism render rational his acceptance of strong scientism? Apparently, he can't, in which case his commitment to it is a matter of irrational ideology.
After all, he cannot appeal to rational insight as a source of knowledge. For that is precisely ruled out as a source of knowledge by scientism.
Scientism falls short of the very standard it enshrines. It is at most an optional philosophical belief unsupported by science. It also has unpalatable consequences which for many of us have the force of counterexamples. So here are some positive considerations against it.
If scientism is true, then none of the following can count as items of knowledge: That torturing children for fun is morally wrong; that setting afire a sleeping bum is morally worse than picking his pockets; that raping a woman is morally worse than merely threatening to rape her; that verbally threatening to commit rape is morally worse than entertaining (with pleasure) the thought of committing rape; that 'ought' implies 'can'; that moral goodness is a higher value than physical strength; that might does not make right; that the punishment must fit the crime; that a proposition and its negation cannot both be true; that what is past was once present; that if A remembers B's experience, then A = B; and so on.
In sum: if there are any purely rational insights into aesthetic, moral, logical, or metaphysical states of affairs, then scientism is false. For the knowledge I get when I see (with the eye of the mind) that the punishment must fit the crime or that a proposition and its negation cannot both be true is not an item of scientific knowledge.