Several of us recently opened up a long back and forth about the nature of truth, and ontological questions more generally. A key part of this conversation was unpacking the pragmatic perspective on meaning.
A nice example that occurred to me later is glass (the material) — specifically the question of what type of thing glass is. I can’t remember exactly where or when, but I’m sure I’ve heard various people claim that glass is in fact a liquid, not a solid. This seems to me like a classic example of an attempted scientific categorization that would defy common sense.
When we think of glass that we are familiar with, we probably think first of its transparency (and tangentially, I wonder if this part of the explanation for what makes people sympathetic to the liquid claim, given that liquids are generally more likely to be transparent than solids), but right after that we likely think of solidity, and brittleness, which are generally antithetical to things we classify as liquids. The claim, however, is that glass actually flows like a liquid — it just flows so slowly that we can’t notice it, or direct it, the way we can with typical liquids.
I’ll revisit this claim in a second, but we should note first that the claim itself is not completely implausible on its face. After all, people have likely encountered substances like molasses, which certainly flows, (it can be poured), and yet is much more viscous than, for example, water. As such, it’s not hard to imagine cranking that knob up much higher, to get a liquid that is so viscous that it barely flows at all; eventually, however, it would take on the shape of whatever container it is in (perhaps over thousands of years).
In addition, we know glass is clearly highly malleable under the right conditions, specifically when it is at very high temperatures; indeed, that is how it can be made to take on such varied forms during manufacturing. So again, if we assume that viscosity decreases with temperature, it could be plausible that viscosity is just very high at room temperature — too high to allow for easy manipulation, but still subtly present.
In the case of glass, it seems like a key piece of evidence that people point to is the fact that very old window panes appear to show a history of flowing over time. In particular, many of them are thicker at the bottom than the top, or contain stream-like patterns. According to Wikipedia, however, there is no solid (sorry!) evidence for this; rather, it seems more likely that such panes were simply manufactured that way, perhaps intentionally or perhaps due to limitations of older manufacturing processes.
Still, we are left with the question of what exactly is a liquid? Pragmatically it seems like a very intuitive category, but as always it is tough to define clear boundaries. We can try to make a definition that is more scientific precise in various ways, which will likely involve describing aspects of material that cannot be observed independent of technology, such as molecular composition. In the end, however, we are faced with the reality that “liquid” and “solid” are not fundamental, pre-existing categories. Rather, they are concepts that humans have come up with (along with “gas”) to describe common, observable, categories — broad basins of attractions that most things we encounter seem to fall into.
If we do make use of some more technical observations, we find that at an atomic level, most things we call liquids have loosely attached molecules that slosh around, allowing them to flow. Most solids, by contrast, have tightly packed molecules, generally in a crystalline lattice, as is the case in metals, minerals, ceramics, ice, etc. It turns out, however, that glass defies this categorization scheme. Glass is rigid and brittle like most solids, but it does not have the lattice structure that is found in most other solids.
As such, various finer distinctions have to be made. Many sources, including wikipedia, now categorize glass as an “amorphous solid”, as opposed to “crystalline solids”. Others have argued that glass should be called a “frozen liquid”. Which is correct? (And how should we arrange the hierarchy of categories?) Well, there may be arguments to be made one way or the other, but again we fool ourselves if we think there is only one, fundamental, scientifically meaningful set of categories that will apply everywhere and always, rather than multiple possibilities which are all compatible with the evidence.
Ironically, if we are left with “flowing” as a defining characteristic of liquids, we run into some more weird conflicts with intuition. It turns out that there is a category in geology called “rheids” which (if I understand this correctly) seems to describe solids — specifically rocks — that flow under pressure. To make matters even worse, this seems to be an absurdly clear case of a category that is defined by a purely arbitrary boundary. According to Wikipedia, “In geology, a rheid /ˈriːɪd/ is a substance whose temperature is below the melting point and whose deformation by viscous flow during the time of observation is at least three orders of magnitude (1,000×) greater than the elastic deformation under the given conditions.” Why three orders of magnitude? No doubt there is some motivation, but I declare it to be arbitrary. Someone chose that number, and it has stuck.
Further down on the page we learn that granite can be categorized as a rheid, meaning that it (like glass?) does flow very slowly over time, seemingly mocking the expression “solid as a rock”.
No doubt I am being somewhat cavalier in talking about many of these concepts, and perhaps experts in any given domain could find ways of aligning their usages such that everything would have a nicer internal coherence. The point, however, is to illustrate the kinds of difficulties we run into when trying to come up with a rigorous and defensible way of carving up the world. Any categorization scheme is like a kind of bet against the universe, daring it to come up with an exception, which it almost certainly will.