There’s a curious thing which happens to those who believe that the only real knowledge comes from science: they start to believe that nearly everything—except what they want to reject—is science. Ultimately this should not be shocking, since people who live with a philosophy will invariably change it—gradually—until it is livable.
The people who become Scientismists generally start out extremely impressed with the clear and convincing nature of the proofs offered in the physical sciences. It would be more accurate to say, with the few best proofs in the physical sciences which are offered to them in school—but the distinction isn’t of great import. In practice, most of the impressive results tend to be in the field of Chemistry. It doesn’t hurt that Chemistry is a bit akin to magic, with the astonishing substances it allows people to make, but what it’s really best at is interesting, counter-intuitive predictions. Physics, at least as presented in school, generally allows you to predict simple things like where a thrown object will land or how far a hockey puck will skid on the ice. These aren’t very practical, and the results tend to be intuitive. Chemistry, by contrast, involves the mixing of strange chemicals with the results ranging from anything to nearly nothing to things which glow to explosions to enormously strong plastics.
And Chemistry does this with astonishing accuracy. If you start with clean reagents and mix them in the appropriate steps, you actually do end up with close to the right amount of what you’re supposed to end up with. If you try to run a physics experiment, you’ll probably be nowhere close to correct simply because the experiments are so darn finicky. I still remember when my high school honors physics class broke into groups to run an experiment to calculate acceleration due to gravity at the earth’s surface. The results were scattered between 2.3m/s and 7.3m/s (the correct answer is 9.8m/s).
The problem for our budding Scientismist is that virtually nothing outside of chemistry and (some of) physics is nearly as susceptible to repeatable experiment on demand. Even biology tends to be far less accommodating (though molecular biology is much closer to chemistry in this regard than the rest of biology is). Once you get beyond biology, things get much worse for the Scientismist; by the time you’re at things like morality, economics, crime & punishment, public decency, parenting and so forth, there aren’t any repeatable controlled experiments which you can (ethically) perform. And even if you were willing to perform unethical controlled experiments, the system involved is so complex that the very act of controlling the experiment (say, by raising a child inside of a box) affects the experiment. So what is the Scientismist to do?
What he should do, of course, is realize that Scientism is folly and give it up. The second best thing to do is to realize that (according to his theory) human beings live in near-complete ignorance and so he has nothing to say on any subject other than the hard sciences. What he actually does is to then declare all sorts of obviously non-scientific things to be science, and then accepts them as knowledge. Which is to say, he makes Scientism livable. It’s neither rational nor honest, but it is inevitable. In this great clash of reality with his ideas, something has to give—and the least painful thing to give up is a rigorous criteria for what is and is not science.