The classic case of a natural kind is water. This natural kind throws up many problems. These problems have been debated many times in — mainly analytic — philosophy. One main focus in this debate has been on the differences between water’s “microscopic” (or “microstructural”) properties and its “macroscopic” properties. More specifically, there’s been some kind of philosophical opposition which has been made between the microstructure of water and its “classical” (or macroscopic) properties. Added to that (though related to macroscopic properties) is the emphasis which has been made on our phenomenological (or phenomenal) experiences of water.
Even those who accept that there are natural kinds still acknowledge that some of the things which are taken to be natural kinds today weren’t taken to be so a hundred years ago (or even more recently). Now facts like that alone don’t give one a reason to be sceptical about the reality of natural kinds. Take mathematics as a similar case. Mathematicians (not just Platonists or realists) believe that there are determinate answers (or “solutions”) to mathematical “problems” even if in the past mathematicians have sometimes got the answers wrong or we don’t have the correct answers today. Similarly, even if we make mistakes about natural kinds; surely they must still be real.
“[I]t is commonly assumed that, among the countless possible types of classifications, one group is privileged. Philosophy refers to such categories as natural kinds. Standard examples of such kinds include fundamental physical particles, chemical elements, and biological species… Candidates for natural kinds can include man-made substances, such as synthetic elements, that can be created in a laboratory…. Groupings that are artificial or arbitrary are not natural; they are invented or imposed on nature. Natural kinds, on the other hand, are not invented, and many assume that scientific investigations should discover them.”
I won’t go into great detail about the definition above because much of it isn’t relevant to what will be discussed later. However, it can be said that it’s certainly the case that as a chemical substance, water (or a sample of H₂O molecules) is “privileged” by… well, (at the very least) philosophers. Yet water is privileged precisely because it’s seen — by philosophers — as being the classic natural kind. So that’s a kind of circular situation.
The passage above also says that natural kinds “are not invented” (i.e., unlike “groupings that are artificial or arbitrary”). There’s a problem here too. What a natural-kind term refers to (or is supposed to refer to) may not be invented. However, the natural kind term itself, and all the definitions which “belong” to it, most certainly are invented. (In a strong sense, this is a case of applying an anti-realist position to natural kinds.) And that may account for the problems we encounter in many discussions of natural kinds — that confusion (or conflation) of what natural kind terms refer to and natural-kind terms (along with their definitions) themselves. What natural-kind terms refer to may certainly be real (at least in most cases). Nonetheless, it’s taking them as natural kinds that’s the (philosophical) problem. And that problem lies at the heart of this piece.
Now considering what was said in the Internet Encyclopaedia of Philosophy definition of the term “natural kinds” above, it may seem strange to consider the possibility that what natural kinds are taken to be may be a somewhat contingent or even arbitrary matter. But, of course, many philosophers will now immediately state that if natural kinds are the result of contingent and/or arbitrary decisions (or even if they have a contingent and/or arbitrary nature), then they can’t be natural kinds at all!
All Samples of Water Contain Micro-Organisms
The philosopher George Bealer picks up on the microscopic-macroscopic-properties distinction (mentioned at the beginning) when it comes to water. He cites the classic twin-earth example of the opposition between XYX-as-water and H₂O-as-water.
Readers must note here that Bealer’s following “thought experiment” is extremely artificial. However, that often doesn’t matter in philosophy because such cases are chosen to illustrate broader philosophical issues. And, in this instance, that broader philosophical issue is the — possibly? — contingent and/or arbitrary nature of natural kinds.
In his paper ‘Propositions’, Bealer firstly cites the possibility (or actuality) that
“all and only water here on earth is composed entirely of certain micro-organisms”.
Even if the factual claim that all samples of water contain “certain micro-organisms” is false, we can still argue that every sample of water will contain at least some constituents other than H₂O molecules — whether that includes “foreign” ions, dust, tiny bits of plastic, or Bealer’s micro-organisms. Now does that matter when it comes to to the nature of the natural kind water?
(It turns out that “nearly every body of water” here on Earth does contain microorganisms — see here. Despite that, one would intuitively believe that chemists must surely be able to create “pure water”. Yet, as it turns out, even chemists can’t do that — see here.)
Bealer states his broader position thus:
“If, like live coral or caviar, all and only water here on earth is composed entirely of certain micro-organisms, then on a twin earth a stuff which contains no micro-organisms whatsoever but which nevertheless contains the same chemicals as those found in samples of water on earth would not qualify as water.”
The general point here is that what we take water to be may be a contingent and/or even an arbitrary matter. Of course Bealer isn’t arguing that water here on earth isn’t H₂O. Or, more correctly, he isn’t arguing that all samples of water don’t contain mostly H₂O molecules. Bealer is saying that the fact that water contains mostly H₂O molecules is only a part of this (possible) story. In his example, literally all samples of water here on earth also contain micro-organisms. Now if every sample of water does contain micro-organisms, then why aren’t these micro-organisms part of the essence of water? Or, alternatively, why aren’t these micro-organisms constitutive of the natural kind water?
Now if we provisionally accept that all samples of water contains micro-organisms, then if what passes for water on Twin Earth doesn’t contain any micro-organisms, then surely it can’t be water. (At least it can’t be water, according to people on earth.) Yet here there seems to be an obvious distinction which can be made here between water (or H₂O molecules) and what else we may find in water. And that’s still the case even if we always find the same given x (other than H₂O molecules) in water.
For example, simply because all trees have fungi, moulds or lichen on them, that doesn’t mean that we have a joint tree-fungi/etc. natural kind. Similarly, if every sample of gold has microscopic particles of dirt on it, that wouldn’t mean that we have a joint gold-dirt natural kind.
The fact that a distinction can be made between water and what else is found in every sample of water may not matter when it comes to Bealer’s central point. That is, why do we (or why do philosophers) exclude these micro-organisms from the natural kind water if they occur (or exist) in literally every sample of water? Having said that, just as all samples of water contain micro-organisms (at least in this case), so every example of water also contains hydrogen atoms. Yet although each H₂O molecule includes two hydrogen atoms, they’re not actually the same thing. Similarly, why conflate micro-organisms and water simply because the former can be found in all samples of the latter?
Let’s look at this another way.
Even if water (or a collection of H₂O molecules) is diluted with whiskey (to invert things for this example), then it’s still water that’s being diluted. Similarly (as earlier), if every tree has fungi/mould/lichen/etc. growing on it, then that doesn’t mean that trees literally are fungi/mould/lichen/etc; or that taken together trees and fungi/mould/lichen/etc. constitute a joint natural kind.
So a summary of Bealer’s argument can be posed in the form of this question:
What if literally every sample of water contains something that isn’t H₂O?
Well, that depends. In one case, a sample of water may contain x and another sample may contain y. So, yes, it may be the case that every sample of water contains something that’s over and above H₂O molecules. But what if water always contains the same x that’s over and above H₂O molecules? That seems to be Bealer’s argument.
Bealer further stresses the contingent and/or arbitrary nature of natural kinds in his Twin Earth example. He continues:
“[T]hen on a twin earth a stuff which contains no micro-organisms whatsoever but which nevertheless contains the same chemicals as those found in samples of water on earth would NOT qualify as water.”
Now we’re in the absurd (or simply possible) situation in which all the samples of a substance that’s entirely made up of H₂O molecules may not be deemed — by people on earth at least — to be water! Why is that? It’s because these samples don’t contain any micro-organisms. Yet intuitively it would seem that the “water” (note the scare quotes) on Twin Earth has more right to be deemed a natural kind (or simply as water) than water on actual Earth. (Still bear in mind the supposition that all samples of water here on earth contain micro-organisms.) Indeed why can’t Twin-Earthers reverse the earthling position by claiming that water on earth is not water precisely because each sample of it contains micro-organisms!
The Microscopic and Macroscopic Properties of Water
Bealer offers us another possibility.
He makes a distinction (mentioned in the introduction) between water’s microscopic (or microstructural) and its macroscopic properties (i.e., rather than between water’s possible different microstructural properties). Bealer writes:
“If every disjoint pair of samples of water here on earth have different microstructural compositions but nevertheless uniform macroscopic properties, then on a twin earth a stuff which has those same macroscopic properties would qualify as water.”
We can of course ask about the chances that “every disjoint pair of samples of water” could have “different microstructural compositions”. Wouldn’t the chances of this be virtually zero? And why choose pairs — rather than triples or n-tuples — of water samples in the first place? The passage above also seems to assume that water can be water even if each member of “every disjoint pair of samples” has a different microstructural composition. In any case, here it’s being argued that microstructure isn’t the only factor to consider when it comes to water’s being water. Indeed if each member of every selected pair of samples contains a different microstructural composition, then microstructure simply can’t be a factor at all!
Bealer stresses water’s macroscopic properties in this example.
The water on Twin Earth has the same macroscopic properties as the water here on Earth. However, is it likely that a chemical substance on Twin Earth with a completely different microstructure would have the same macroscopic properties as water here on Earth? Well, it is of course possible. That is, I’m assuming here that Bealer is at least partly referring to experiential (or phenomenal) properties — such as water’s transparency, wetness, liquidity, variant temperature (as registered by the sensory-systems of human beings), thirst-quenching qualities, etc. (The properties of water which chemists cite are very different to these. They include polarity, surface tension, cohesion, adhesion, evaporative cooling, etc.) Now could something that isn’t made up of H₂O molecules be wet, transparent, quench thirst, etc. in exactly the same way that water here on Earth is and does? Well, as before, I presume that all this is possible. (What biological or physiological effects would Twin-Earth water have if a earthling drank it?)
So now we can sum up Bealer’s position with another simple question:
When it comes to natural kinds, why shouldn’t macroscopic properties (i.e., rather than exclusively microstructural properties) be what is important?
Again, isn’t it somewhat arbitrary and/or contingent that philosophers see only microstructural properties as being constitutive of natural kinds (i.e., at least when it comes to natural kinds like water), rather than seeing macroscopic properties in the same way?
Are There Different Kinds of Water?
One needn’t be a chemist or a layperson to find the position that “there are other kinds of water” odd (as some philosophers have done). More concretely, if these other kinds of water share nothing with H₂O molecules, then why are they water at all? Alternatively put, is it possible that “not all water has the same microstructure”? All that may depend on what’s meant by the words “share nothing”. For example, in one scenario it is the case that all rival samples of water do share macroscopic properties; though not microstructural properties. (We can of course debate how the sharing of macroscopic properties actually cashes out.)
“[I]ndeed, we knew this already, since it would surely be stipulative to deny that heavy water (D₂O) is really water.”
Here we’re back to our contingent and/or arbitrary (i.e., “stipulative”) decisions concerning natural kinds. After all, it’s quite possible that some chemists don’t see D₂O (or “heavy water”) as water. In other words, what’s to stop them from deciding that D₂O isn’t water? More clearly and obviously, if D₂O isn’t H₂O, then surely D₂O and H₂O can’t both be water. However, it is the case that the molecules H₂O and D₂O do share some things — they both include an oxygen atom, protons, electrons and other chemical/atomic elements/forces. But does all that matter? Is all that enough?
(A D₂O molecule includes a “heavy” hydrogen atom. It’s heavy because it contains an extra neutron in its nucleus, along with the standard proton. The light hydrogen atom, on the other hand, only contains a single proton.)
Of course one way to “solve” this particular problem is simply to see water’s macroscopic properties as being constitutive of it being a natural kind. Thus, in this case at least (as stated), both H₂O and D₂O do have exactly the same macroscopic properties! (Or do they? Yes; H₂O and D₂O have the same macroscopic properties when it comes to the sensory — or phenomenal — experiences of human beings. But they don’t do so when it comes to chemical analysis — see here.)
So if D₂O is water, then why can’t Twin Earth’s XYZ (along with H₂O and D₂O) also be water? After all, XYX does, at least hypothetically, have the same macroscopic properties. (At least it’s taken to do so in the philosophical literature.)
It’s precisely because of these problems that some philosophers have argued that the word “water” is not a natural kind term at all. However, that may well still mean that the symbol “H₂O” itself does actually symbolise a natural kind. So what natural kind does symbol “H₂O” symbolise? Water? Perhaps, then, in order to avoid this circularity all we really have left is this:
The symbol “H₂O” = (or refers to) H₂O
(Of course the mathematical identity/equality sign above can’t be taken literally. A symbol can’t literally be identical to what it symbolises.)
Or, at the the very least, all we have is this:
The symbol “H₂O” = a molecule made up of two hydrogen atoms and one oxygen atom (plus lots of other molecular, quantum, bonding, etc. stuff)
So is heavy water (or D₂O) a genuine natural kind? And if we take H₂O as a natural kind, then are both D₂O and H₂O genuine natural kinds? Indeed are they the same natural kind (if with slight microstructural differences)? But, again, which one is truly water? Both? Alternatively, perhaps neither is a natural kind.