One label that often gets applied to me is “materialist”, or sometimes “physicalist.” It’s a label that, while it probably gives an accurate idea of my conception of reality, I’ve generally resisted. Why? Because if there were ever any evidence for anything non-physical, I would accept its existence. Consequently, I’ve often felt that a better label for me was “evidentialist.”
I’ve given this reply in multiple conversations. One person challenged me to clarify what I meant by “evidence.” It’s a good question. I generally mean empirical evidence that is reproducible or otherwise verifiable. But, the challenger asked, am I not stacking the deck here? Isn’t requiring physical evidence somewhat circular? If the only thing I will accept for the non-physical is physical evidence, then haven’t I put myself in a closed loop?
Part of the problem here is that if we ever did encounter a physical event whose causes could not be accounted for by other physical events, it’s very unlikely that we’d assume that they’d been caused by a non-physical event, such as perhaps pure mental activity. We’re much more likely to assume that we simply don’t yet understand all of the physics involved.
Look at our response to situations like our inability to account for the rotational speed of galaxies, or the expansion rate of the universe. We don’t know what the physical causes of these phenomena are. We hang labels on them: “dark matter” and “dark energy”, but those terms are really just placeholders for our ignorance, an epistemic hole that we can work around mathematically but can’t yet enter.
It’s often now forgotten that gravitation was once just such a placeholder. When Isaac Newton first worked out his laws, he really had no idea what gravitation actually is. It was really just a label applied to something whose effects on matter we could observe and model, but that we couldn’t directly understand.
Some scientists at the time were hesitant to accept his theory, mainly because it appeared to bring back something they had discarded with the onset of the mechanistic philosophy in the 16th century. This philosophy, which was basically proto-materialism, posited that there were no actions at a distance. Phenomena could only affect other phenomena that were local to it. But gravitation seemed to be exactly that. Eventually though, our conception of materialism was expanded to accept it.
Of course, much later, Albert Einstein came along and demonstrated that gravitation was the warping of spacetime, and that changes in that warping propagated at the speed of light, restoring locality. (An understanding that just recently received additional confirmation with the discovery of gravitational waves.) But for centuries, it was simply taken on faith that there would eventually be a mechanistic type of explanation for what was happening.
Or consider the epistemic issues involved with quantum physics. It’s fair to say that there are things going on in wave particle duality that we simply don’t understand. There are many interpretations of what is happening, but none have unique evidence to back them up, at least not yet. All of the interpretations have to give up some primal aspect of how the physical world is understood to work. Some in particular sacrifice the locality that Newton’s contemporaries were loathe to surrender.
But this is aside from the fact that there is nothing that says that empirical evidence must by physical. Empirical evidence is, when you get right down to it, simply conscious sensory perception. This is why reproducibility or verifiability is so important before counting it as evidence.
I have a sensory perception of the desk in front of me. I’m confident that there is a desk there because I’ve experienced it many times (and bumped my head against the underside of it once or twice) along with many other people. That the desk exists is an ontological theory, but it’s one that I have extensive evidence for.
But then, what does it mean to say that the desk physically exists? Fundamentally, what does the word “physical” mean? It seems to mean that which exists objectively, that is, that which is independent of our minds? It appears to be there even when we’re not thinking about it or don’t expect it (as when I bumped my head against it).
A mathematical platonist might argue that mathematical realities also fit this category, but aren’t included in what we normally mean by the word “physical.” But the ontology of mathematical objects remains a debated matter, and I’m not personally a Platonist, being more of a mathematical semi-empiricist, believing that mathematical structures are either a description of physical relationships, or are imagined models we hold, derived from those structures that are descriptions of reality, but with added non-real elements, or missing real ones.
Perhaps another way of looking at this is that what is physical is that which is included in the physical causal framework, that can have some effect on other physical phenomena. But this feels dangerously vulnerable to the circular criticism above. And I’m not sure how some quantum interpretations might fit in it.
But is the circular criticism fair? If there is a non-physical reality, but it can’t have any effect on the physical portions, does it exist for us in any meaningful sense?
Going to the heart of the real issue here, if there is a non-physical spirit or soul which can’t have any effect on the physical brain, does it really exist in any meaningful sense? And if we did discover that there were phenomena going on in the brain whose causes we couldn’t account for, would we assume those causes were non-physical spirit? (No doubt some would, but I’m talking about scientists here.)
Or would we do the same thing we do with dark energy, dark matter, and once did with gravitation? Assume that we simply don’t have a full understanding yet, and keep investigating.
In other words, if there is a non-physical reality, how would we go about objectively ascertaining it? Is that even a meaningful question? Why or why not?