Keith’s Conundrums: Is Water Wet?

Keith Douglas

Last time I wrote about professionalization in the context of computing in general and AI in specific. I received no comments. For this month, the editor suggested I investigate wetness and in particular, “Is water wet?”

There are a lot of websites out there — let’s use ScienceLine for our pruposes — that discuss this matter. There seems to be a consensus that water makes things wet, but it is not itself wet. Let’s look at an example from ScienceLine and from the perspective of a metaphysician who remembers some elementary chemistry and physics. I start with a quotation (emphases throughout are in the original).

“Wetness is the ability of a liquid to adhere to the surface of a solid, so when we say that something is wet, we mean that the liquid is sticking to the surface of a material.”

The first clause is an assertion that wetness is a property. This sounds innocuous enough and seems plausible to me. The next clause unpacks “wet,” however. This seems to suggest that something is wet when the property of wetness is, so to say, activated. What would be a parallel to this? Solubility and solution? (Is a solution a single thing?) This is not the whole story, as the explanation continues.

“Whether an object is wet or dry depends on a balance between cohesive and adhesive forces. Cohesive forces are attractive forces within the liquid that cause the molecules in the liquid to prefer to stick together. Cohesive forces are also responsible for surface tension. If the cohesive forces are very strong, then the liquid molecules really like to stay close together and they won’t spread out on the surface of an object very much. On the contrary, adhesive forces are the attractive forces between the liquid and the surface of the material. If the adhesive forces are strong, then the liquid will try and spread out onto the surface as much as possible.”

This adds another aspect to the characterization. There are some anthropomorphisms that should be ignored here; this is somewhat harmless in this context. I would think a science education site would be wary of such a thing anyway, however. We then continue.

“So how wet a surface is depends on the balance between these two forces. If the adhesive forces (liquid-solid) are bigger than the cohesive forces (liquid-liquid), we say the material becomes wet, and the liquid tends to spread out to maximize contact with the surface. On the other hand, if the adhesive forces (liquid-solid) are smaller than the cohesive forces (liquid-liquid), we say the material is dry, and the liquid tends to bead-up into a spherical drop and tries to minimize the contact with the surface.

Water actually has pretty high cohesive forces due to hydrogen bonding, and so is not as good at wetting surfaces as some liquids such as acetone or alcohols. However, water does wet certain surfaces like glass for example. Adding detergents can make water better at wetting by lowering the cohesive forces. Water resistant materials such as Gore-tex fabric is made of material that is hydrophobic (water repellent) and so the cohesive forces within the water (liquid-liquid) are much stronger than the adhesive force (liquid-solid) and water tends to bead-up on the outside of the material and you stay dry.”

This is additional information as well. It makes it clear that the property of being wet is of sufficiently extended surfaces. But note the unusual characterization of “dry.” Is this a distinction without a difference? There is some amount of water in the air at all times, and some of it would be at any moment evaporating from or condensing on surfaces. So all surfaces on Earth would be to some degree wet to the extent they are exposed by the atmosphere.

There is another part of this description of metaphysical interest. Adhesive forces (those between the liquid and the surface) are relational properties: They are not intrinsic to a liquid. So there is another sense in which water cannot be wet: It takes two to tango! (And to think that, not even 150 years ago, most philosophers struggled with relational properties and to this day some think they are suspect.)

So we conclude that part of the puzzle is that in ordinary language we usually do not distinguish between wetness and being wet. However, there is still another twist, which the second answer on ScienceLine discusses: In ordinary language we often fail to distinguish between sensations — which it can be argued are relational properties between the physical environment and our nervous systems — versus the physical properties of the environment.

For example, “loudness” is attributed to sounds. In fact, sounds themselves are often taken in this way. This is not how the term is used in the branch of acoustics. Be that as it may, intensity is what is sometimes meant and sometimes our reaction to it. For some domains, the distinction is not reflected at all in language. There is no term for “subjective wetness.” The sense of wet is also a relational property, this time a three-way one, as the organism is involved, and thus water is not wet on this understanding either.

Incidentally, the distinction here (subjective/objective) answers the old question: “If a tree falls in the forest and nobody is around, does it make a sound?” The answer is “Yes, if sound is taken to mean pressure waves in the air, the way it is in physics.” It is also, “No, if sound is taken to be the psychophysical response to pressure waves in the air by animals, etc.”

An interesting philosophy-of-science lesson can also be drawn: Single sentences (or single propositions) do not make a scientific theory, contrary to much ordinary language and some usage in some scientific contexts. Note that there are several here to form an informal theory of wet. (This is how the term is used in mathematics and theoretical physics, for example.)

Finally, note that contrary to some philosophers’ views, one can objectively study subjectivity. Ironically, one of the pioneers in this area, Ernst Mach, was a subjectivist. His little book, Contributions to the Analysis of the Sensations, makes the bizarre claim that the world is made of sensations, and so we should study those, rather than study the supposedly independent world itself.

Next time: I do take requests from the readers (not just the editor), if people are interested. Use the comments if you wish to do this.