What's the distinction?
I think you should probably define your terms. What do you mean by "science"? Is it experimentation? Empirical observation?
I would say that empirical observation is the best way to know that something is measurable and reliable. Or predictable, if you prefer. Or explanatory.
The way I see it, "science" in this context means the scientific method: Observe, predict, test, observe. Is there another way to know if a claim is true or real? I don't think so. Is there another way to demonstrate to others that a claim is true or real? I don't think so.
If you think there might be another way, I'd expect you to describe it, and I'd expect you to demonstrate that it works.
In mathematics, the process is the same. The hypothesis is called a conjecture, and the test is called a proof. The conjecture arises from logical inferences based on empirical observation of the mathematical rules of logic in action.
For example, observation of the behavior of knots, according to the rules of four-dimensional manifolds, leads to the conjecture (hypothesis) that the Conway knot is slice. For many years, no proof (test) of this conjecture was known. Recently, mathematician Lisa Piccirillo devised a proof that the Conway knot is in fact not slice. Now we know. This is the essence of the scientific method. Observe, predict, test, observe.
We can even consider your question in terms of the scientific method. We can rephrase the question as a hypothesis: There is another way (besides the scientific method) to know something is true. If we do this, we see that a few steps are still missing. Your hypothesis doesn't seem to arise from any observations. What do you see, that leads you to think there might be other ways to know if something is true or real? And your hypothesis so far lacks a test. What do you propose, to test your hypothesis? Finally, without a test, we have no observation of whether the hypothesis is true or not.
- Observe
- The scientific method is not the only way to know if something is true or real.
- Test
- Observe
So. According to the scientific method, your hypothesis is currently unproven, and currently untestable. We'll never know for sure if it's true or not, but that's okay. The scientific method doesn't concern itself with ultimate truth or reality, only observable truth and reality.
Maybe your hypothesis is true. Science could tell us, if you were to devise a test.
Maybe some other method could tell us if your hypothesis is true. But until you describe and demonstrate such a method, we won't know.
Meanwhile, what do you mean by "scientism"? I would say scientism is the belief that science is the only way to know whether something is true or real.
This brings us back to what I mean by science. By science, I mean the scientific method: observe, predict, test, observe.
I would say that the scientific method (a.k.a. science) is the only method that we know of, to determine whether something is true or real. Therefore, I think scientism is justified - it is a reasonable working assumption. Of course, without a testable hypothesis or observable results, we'll never know for sure. But that's okay. Not knowing for sure is also part of the scientific method.
So that's where I'm at, with your question. Is scientism justified? Yeah, I think it is. If you think it isn't, that's fine. If you want me to think it isn't, then I expect you to come up with observations, predictions, and tests, that show me it isn't. Or come up with some other way to convince me.
Here's how I would structure the problem, scientifically:
- I observe that people like to get rich.
- Hypothesis - if there's some other way to know if things are true and real, someone would be using it to get rich.
- The way to test this is to see if people are using other ways to get rich, besides prediction, testing, and observation.
- I don't observe people using other ways to get rich.
So my scientific conclusion is that there are no other ways to know if something is true or real - that we know of. Scientism is justified so far.