In a discussion a while back I mentioned in passing that humans sense wetness and time. That was challenged on the basis that we don’t sense time at all and — when it comes to wetness — sense only pressure and temperature. There is some truth to that. We don’t have an actual time sensor, nor do we have specific “wetness” sensors.
I’ve been thinking about this ever since (not constantly; you know, on and off). A key question is whether wetness can be reduced to pressure and temperature and remain wetness. And time is a topic all on its own!
For the record: Here is my final answer…
The first thing is that the property of wetness is itself an emergent property of liquid water. Steam and ice are not wet. An H2o molecule has no “wetness” property. It does have some electrical properties that cause groups of H2o molecules to behave in a way known as wetting.
We experience many emergent phenomenon:
- A movie (or a video) is a series of still pictures(frames) seen fast enough to appear to us as motion (hence the term, motion pictures).
- A musical chord emerges from simultaneous notes; a melody emerges from notes played successively in time.
- The pixels that make up the image on any monitor or display screen are little dots of red, green and blue. White, yellow, cyan, and magenta, are emergent colors.
If we separate the colors or the notes or the pictures, we no longer perceive the emergent phenomenon. They are irreducible. They exist only in the sum of the parts.
A characteristic of these emergent phenomenon is that what emerges does so only in our minds. There do exist emergent phenomenon where the emergent property is physically measurable. Air pressure is an emergent property of the motion of air molecules. It can be measured with a gauge.
But the motion of a movie, or harmony of a chord, or the perception of white from red + green + blue, cannot be measured with any gauge. They are integrated and perceived within the human mind. Even a shift of mental focus, such as with some optical illusions, can change our perception.
Is wetness also a set of sensory inputs integrated and perceived only in the human mind? As with musical chords, wetness has physical properties that can be measured. Wetting is a specific and quantifiable process we can characterize objectively. But the sensation of wetness, as with the sensation of a chord, may be a type of qualia — something experienced only by minds.
The wetness of water seems more than just a temperature and a pressure. Water has a vast range of temperatures, and many not-wet things have temperatures in that range. Given the range water does have, temperature doesn’t describe wetness very well.
Water does have some unique temperature characteristics. It’s a huge heat sink — it can absorb and hold heat like crazy. Your hand in any decent amount of water doesn’t affect the water temperature, so you feel constant temperature. Water cools what it evaporates from. If your arm is wet, a good breeze chills it.
That heavy close feeling of high humidity comes from water vapor in the air blocking evaporation from your skin. You sense water’s heat behaviors on that subtle of a level!
These unique temperature characteristics may be a big part of the integrated sensation of water. (For that matter, even the electrostatic properties involves in wetting may be sensed by your nerves in ways below your perceptual horizon.)
Pressure is more specific. One characteristic of water is that it touches your skin smoothly everywhere. In fact, specifically, it wets your skin. That’s a fairly unique sensation. Many dry things have a surface texture water lacks. (The viscosity of water may also play a role in the sensation of wetness.)
Slick rubber, leather, or metal, can sometimes briefly fool us by feeling wet. A common response having touched such an object is to rub thumb and finger to see if a surface of water is present. Another characteristic of wetness is that it transfers to what touches it.
My bottom line, though, is this: Is it possible to describe “wetness” in terms of temperature and pressure in a way that is both descriptive and unique? Can it be reduced to a description of temperature and pressure or does that leave something out?
The wide range of water temperatures is so generic. Pressure and texture might describe more, but I think that — as with the canonical seeing red — wetness is an irreducible sensation (a qualia) you have to experience to know.
What about time?
Maybe that’s something to do in more detail another time. For now just consider what it means to have a thought. That is, for your mind to process a serial set of symbols.
In order to have the thought, “I think, therefore I am!” there must be time in which to have the thought.
Your sense of balance is based on “sand” in little hollow donuts inside your head. You don’t sense the sand moving around inside the donuts. You do sense your balance. (There are more than the five physical senses.)
There may be no physical time sensor in the human body, but the biological processes of our bodies occurs in time, and our mind occurs in time, so it seems to me we have a very definite (if non-physical) sense of time.
So it’s been interesting to chew on, but I think I’m going to go with my original answer: Humans sense wetness and time. The former is an irreducible qualia perceived in the mind (yet also having distinct and complex physical properties; see tree, forest, sound). The latter is a non-physical sense that also occurs in the mind.
I do think it is very interesting to pay attention to qualia, to tease them into what parts we can discern, but I’m not sure reducing them to those parts keeps the whole. It’s like focusing on the pixels.
 Anyone who’s lived in really cold country — with temps down to -20 (F) for days in a row — knows that ice is rock-hard and dry as a bone at those temps.
For example, the water ice on Pluto isn’t just rock hard, it is rock! And it’s no more “wet” than a rock you’d pick up off an Earth mountain would be. (Assuming it wasn’t raining or something.)
Steam isn’t wet, either, but when it hits something that cools it down, condenses to liquid water, which makes the surface wet.
 For that matter, seeing an image at all from a set of dots is an emergent thing.
 This one is, perhaps, debatable. The beat notes and harmonics created by a chord are physically present. It’s really a version of the “tree falls in the forest” question. It depends on how you define sound: pressure waves or what is heard.
But ask yourself why different musical intervals (e.g. a fifth versus a seventh) please the ear differently or why different chords (e.g. C versus Cm7) sound different. That reaction is human.
 Being in colder water always sucks heat from your body. Being in hotter water always heats you up. The water always wins. This is why falling into icy water is a killer.
 The “offset” (lithographic) printing industry depends on this not being true in all cases. Specifically, water and oil don’t mix.