Let’s define spectaclism as the theory that what exists is what my spectacles “tell me” exists, i.e. what I am able to see using spectacles. Naturalism is the theory that what exists is what the natural sciences “tell us” exists, i.e. what we are able to learn via their methods.
In support of their theory, naturalists point to the many predictive and technological successes of the natural sciences. In favor of their own theory, spectaclists could also point to the great predictive and technological accomplishments of spectacle-wearers.
Still, no one believes in spectaclism, and the reason is obvious: That a certain method provides us with reliable and useful information about some domain gives us no reason whatsoever to think that what it tells us exists is all that exists. There are other problems too: What sorts of spectacles are the ones we should rely on to tell us what exists? Bifocals? Sunglasses? What color? And why those, exactly?
Notice that the problem here is a failure to keep in mind that metaphysical questions are prior to epistemological or methodological ones. You have to determine first what exists before you can find out whether spectacles tell you all there is to know about it, exactly which spectacles do the job, etc. Spectaclism, in short, gets things back-asswards.
But here’s the thing: Naturalism is exactly as back-asswards as spectaclism is. It is silly to suggest that what exists is only what natural science tells you exists unless you already know through independent means what exists, can compare the deliverances of all putative natural sciences to it, and determine on that basis that such-and-such putative natural sciences alone capture everything there is.
Of course, naturalists will tell us that there is no alternative to natural science – that common sense perceptual experience, introspection, putative religious experiences, and metaphysical inquiry are trustworthy only to the extent that they are vindicated by the natural sciences. But spectaclists could say something similar: Putative alternative sources of knowledge are to be trusted only to the extent that they can be given a respectable spectaclist foundation. “But that’s ridiculous!” Sure it is. So is naturalism.
“Oh come on, we have independent grounds for holding that more exists than is dreamed of in the spectaclist’s philosophy!” Sure we do, but we also have independent grounds for holding that more exists than is dreamed of in the naturalist’s philosophy – for example, grounds derived from common sense perceptual experience, introspection, putative religious experiences, and metaphysical inquiry. “But those aren’t reliable sources of knowledge!” Oh, you mean the way non-spectaclist sources are not reliable? What’s the difference, exactly? And try not to beg the question this time.
“But, but, but, but… naturalism just can’t be as groundless as that!” Wanna bet?
“But most contemporary academic philosophers are naturalists! How can they all be wrong?” Tsk tsk, come now, everyone knows that arguments from authority went out with the Middle Ages…