There is no consensus on a census of the senses.
When we talk about human senses, we traditionally assume that there are exactly five senses — sight, hearing, taste, smell, and touch. This way of thinking about the senses is quite ancient, dating back more than 2000 years. On the assumption that this model is factually correct, we teach “the five senses” to our children from a very early age. This model is so ingrained in our culture that any additional method of perception, whether real or imagined, is usually called “a sixth sense”.
However, there are serious weaknesses in our traditional model of five senses. By any objective measure, humans actually possess more than five senses. Of all the basic scientific models that we traditionally teach our children, few deviate from reality as blatantly as our model of the five senses. That’s not to say that the model is completely worthless. Because the model is so simple, it is easily learned, even by very young children. Therefore it can serve as a helpful framework for early learning. But for older children and adults, the model seriously constrains our thinking about the senses.
A principal characteristic of the five-sense model — and one reason why it is so appealing — is that each of the senses is paired with a unique and highly visible part of the body — eyes, ears, mouth, nose, and skin. In fact, this way of thinking is actually a model of our five most obvious sense organs, rather than a proper model of the senses, and this is what makes it ideal for teaching to preschoolers — in conjunction with learning to identify and name the major parts of the head and body.
Unfortunately, there is no universal agreement as to how many senses humans actually have. The main difficulty is that the count can vary considerably depending upon how you define the word “sense”. Another problem is that as you add more senses to the list, the boundaries between the senses become more blurry, and therefore the count depends upon where you decide to draw the boundaries. Another factor is that some animals possess senses that humans do not — such as the ability to detect magnetic fields. (A thorough discussion of the senses should probably take into account all animals, not just humans.) For all of these reasons, experts disagree as to how many senses there actually are. Without a general consensus as to what model should replace the 5-sense model, the old model retains its strong popularity. That said, a 9-sense model (discussed later in this essay) is probably the strongest contender for replacing it.
One key characteristic of the 5-sense model is that all of the senses are related to detecting phenomena that originate outside of our bodies. In other words, the five traditional sense organs are all tools for investigating the world around us. We see, hear, smell, taste, and touch the things that surround us. If we limit our count of senses to those that detect external phenomena, then our count will never get very long — although it will indeed be more than five. One helpful approach is to itemize the categories of detectable phenomena that originate outside the body:
1) Light (electromagnetic radiation)
Our eyes detect light, or more precisely, they detect a limited range of frequencies in the spectrum of electromagnetic radiation. But equally important, the lens in each eye focuses images on the retina, which allows us to deduce the precise shapes and locations of objects that reflect or emit light. The four kinds of photoreceptors in our eyes (rods and three kinds of cones) allow us to distinguish between frequencies of light, which the brain perceives as color. The fact that we have two eyes with overlapping fields of vision provides us with the ability to judge distances.
Several kinds of animals, including birds and bees, have the ability to see frequencies of light into the ultraviolet, which humans cannot see. Certain kinds of snakes can detect infrared light using “pit organs” in their heads — allowing them to detect the body heat of their prey. However, the sensors in these pit organs work by detecting subtle temperature changes in the tissue lining the pits, rather than directly detecting the photons of infrared light.
As with all of the senses, detecting something with the sense organs is only the first step. The information then needs to be relayed to the brain via nerve pathways, and the brain assembles and interprets the information to produce our perception of the sense. It is our brain that sees patterns, colors, and movement in the data sent from the eyes. It is our brain — not our eyes — that picks out faces in a crowd or in a photograph.
2) Sound waves (vibrations)
Our ears detect sound waves in the air, within a certain range of frequencies. Although we cannot hear sounds whose frequencies lie outside that range, we are very good at distinguishing between the audible frequencies, and at distinguishing other characteristics of sounds. Because we have two ears, we have a sense of what direction a sound is coming from. These abilities not only help us to detect what is happening in the world around us, but also allow us to communicate with other humans through speech.
Some animals are skilled at detecting vibrations in other media besides air. Animals that live in water will, of course, detect sounds waves in water. Other animals can detect vibrations in more solid objects. For example, an insect trapped in a spider web sets up vibrations that not only alert the spider, but also tell the spider certain details about what has been caught. Many kinds of animals, including elephants, can detect and interpret vibrations coming through the ground.
Some animals, such as bats, have developed the ability to “see” their surroundings through echo-location. This means that they can determine the locations and shapes of nearby objects by detecting sounds waves bouncing off of them — somewhat analogous to our own ability to assemble a mental image of the world around us by observing reflected light.
3) Odors & flavors (chemical molecules)
Our senses of smell and taste are both based on detecting molecules of various substances that come in contact with our bodies. In the case of smell, we use the nose to detect airborne molecules of materials — in other words, substances that have evaporated into the air. In the case of taste, we detect five distinct categories of molecules that are present in our food — or in anything else we put into our mouths.
Our perception of taste is due to input from both of these senses. The taste buds on the tongue detect molecules that are sweet, sour, salty, bitter, and savory — but all of the other flavors we detect in our food are due to the molecules that reach the nose. The mouth and nose are connected to each other by passageways at the back of the throat. As we chew our food, we release volatile molecules that waft up through this connection into the nose. In contrast to the five distinct types of taste buds, the nose includes around 400 distinct olfactory receptors. These 400 sensations result in millions of possible combinations, allowing us to detect up to millions of distinct odors.
We all know that are many other animals besides humans that can smell with their noses and taste with their mouths. The surprise is that certain creatures can taste or smell with other sense organs. Some insects can detect airborne molecules with their antennae — meaning that they use their antennae to smell. Some insects can detect molecules in materials that they touch with their feet, meaning that they have a sense of taste in their feet.
4) Direct contact (touch or pressure)
We have several distinct kinds of receptors in our skin, one type of which specializes in detecting touch or pressure. This allows us to determine when our body has come into contact with an external object. Although these receptors are in all parts of our skin, the density of the receptors varies considerably. In other words, in some parts of our skin — such as our hands — a large quantity of receptors are packed into a small area, giving those parts of our skin a much better ability to gather information and to discern shapes, sizes, and textures.
Our hands have a second advantage compared to other parts of our skin. The flexibility of our hands allows us to explore surfaces in much more detail. With our eyes closed, we can easily determine the shape and size of a small object just by touching it with our hands. This is very hard to do with any other part of our skin. Part of the trick is that we don’t have to feel the entire surface at once. We can spend several seconds feeling different parts of the surface, and then our brain puts the information together. So in our traditional 5-sense model, we could have associated the sense of touch with hands, rather than skin — there are good arguments both ways.
The sense of touch can be extended over some distance by the use of a long, slender appendage, such as the whiskers of a cat or the feelers of an insect or crustacean. In the case of a cat’s whiskers, the touch receptors are located in the skin surrounding the base of the whisker. But in the case of a feeler (an antenna used for touching), the touch receptors are actually located in the feeler. In many cases, the same antennae contain other kinds of sense receptors, allowing for smell, taste, hearing or other capabilities.
5) Heat & cold (temperature)
Another type of receptor in our skin is one that detects changes in temperature — the hot and cold receptors. Although this type of receptor provides us with information about the world around us, it does so indirectly — because these receptors do not directly sense the outside world. Instead, they detect temperature changes in the skin. The skin, in turn, is heated or cooled by contact with the air or other objects, and also by exchange of radiant energy (primarily infrared radiation). The upshot is that when we feel the heat of a fire, it is not by directly detecting the radiant energy striking the skin, but by detecting the resulting change in the temperature of the skin. (The main difference between our temperature sense and the pit organs in a snake — other than the degree of sensitivity — is that the pit organs allow the snake to more accurately pinpoint the direction from which the heat originates.)
In the 5-sense model, the sense of hot and cold is completely ignored, or else it is bundled into the sense of touch, even though it is a very different sense. After all, you don’t have to touch the sun in order to feel its heat!
6) Gravity & acceleration
Our ability to detect gravity and acceleration is usually called our “sense of balance”. For this we rely upon the semi-circular canals in our inner ears. Even though gravity is a phenomenon that originates outside of our bodies, the only thing we learn from detecting it is which way is “up”, which allows us to maintain our bodies in an upright position as we stand or walk — even when our eyes are closed.
This is a very real sense, with an easily identified sense organ. And yet this sense is not included in our traditional 5-sense model — in part because the sense organ is not visible on the outside of the body, and in part because the 5-sense model predates our understanding of the role of the semi-circular canals.
Although mammals rely on their semi-circular canals to provide a sense of balance, many invertebrates use a very different organ called a statocyst. In either case, the purpose is to detect gravity in order to know which way is up, so that the body can be properly oriented for safety or locomotion.
7) Magnetic fields
Many kinds of animals are able to detect magnetic fields, even though humans cannot. This gives them an ability to detect the earth’s magnetic field, which can result in a powerful sense of direction (especially north and south). The best-known examples of this phenomenon are birds that fly long distances for their spring and autumn migrations.
A sense organ that detects magnetic fields can be compared to a compass. However, the individual receptors can be extremely small, and could theoretically be anywhere in the body, even in the brain itself. The upshot is that while we have excellent evidence that many kinds of animals have a magnetic sense of direction, in most cases we are not sure exactly where the magnetic receptors are located.
8) Electrical fields and static charge
Some aquatic animals have the ability to sense changes in the electric field in their immediate vicinity. The best-known examples are sharks and rays, but certain other sea animals also have this ability, including dolphins. This sense can be used to identify prey and other nearby objects, which can be quite useful when the water is murky or dark, or when the prey is hiding in the mud or silt on the seafloor.
For animals that live surrounded by air instead of water, the direct sensing of electric fields is not possible. However, some animals — even humans — can detect static charges through indirect means. In the case of humans, a nearby static charge will cause the hair on our arms to stand up, which we can easily feel. Of course, we can also feel gusts of wind using the hairs on our arms. The receptors surrounding the hairs cannot distinguish between these two phenomena — but our brains, upon receiving the information from many hair follicles over a period of several seconds, can easily distinguish the two. This ability should be categorized as an extension to our sense of touch — like the whiskers of a cat — rather than a separate sense. In contrast, sharks really do have an additional sense for directly detecting electrical fields.
Let’s pause here and take stock of our list so far. We have identified eight detectable external phenomena — nine if you separate airborne molecules (smell) from non-airborne (taste) — and every one of these phenomena corresponds to a specific sense in various animals. Humans have seven of these nine senses, lacking only the ability to detect magnetic fields and electric fields. Therefore, any new model of the senses should list at least 7 senses (if we consider only humans) or 9 senses (if we consider all animals). If we define the word “sense” to mean only the detection of external phenomena, then our count is finished: There are 7 human senses, and 9 principal senses across the animal kingdom.
However, our bodies have additional sense receptors beyond the ones we have catalogued so far. Instead of telling us about phenomena that are external to our bodies, these additional sensors provide information about our own bodies. The most obvious example is our sense of pain, triggered by pain receptors located not just in our skin, but also deeper within our bodies. (Broken bones and other internal injuries can be quite painful.) But we are also aware of other internal phenomena, such as being hungry or thirsty, or feeling too full from having eaten too much, or having a full bladder and needing to go to the bathroom. All of these require some sort of sensor within the body in order to detect the issue. The sensors, in turn, send messages to the brain via the nervous system. Therefore we could legitimately refer to a sense of hunger, a sense of thirst, or a sense of being full. In fact, scientists have catalogued a long list of such internal senses. If we were to include all of these senses in our list, then we could easily reach 20 or more distinct human senses.
A subtle but important sense that has gotten a lot of press recently is called proprioception. This is the sense of knowing how the various parts of your body are positioned, without relying on sight or touch. A demonstration of this sense is to close your eyes, and then to reach up and touch your nose. Most people can do this quite easily. Several recent articles in the popular press have stated that because of this newly recognized sense, we now know that humans actually have six senses instead of five. This is obviously incorrect, because if we were to agree on a new model of the senses to teach in our schools, then the senses of balance, temperature, and pain are all stronger candidates for inclusion than proprioception. That said, proprioception is certainly a valid candidate, and ought to be considered.
A related issue is what terminology to use when teaching the senses to older children. We know to use very simple terminology when teaching preschoolers, but as kids grow older, we have a tendency to introduce more complex terminology — some of which is rather pointless. For example, there is little value in teaching children to say “audioception” in place of “sense of hearing”, or “gustation” instead of “sense of taste”. By the same token, the formal term “proprioception” simply gets in the way of teaching kids about the corresponding sense. It would be more appropriate to use an everyday term that conveys the underlying concept in an easily understood manner.
So what really is the underlying concept for proprioception, expressed in a single word? Some people explain proprioception as knowing the location of one’s limbs — but a “sense of location” would be a highly misleading phrase. Furthermore, the receptors in our muscles, tendons, and joints do not actually sense the location of our limbs in space. Instead, these receptors detect the degree to which the muscles are flexed and the angles of the joints, which allows the brain to deduce the position of the body and the position of each of the limbs. Therefore the best term for this sense, at least for teaching children, is “a sense of position”.
So imagine if we were all to agree on a new model of the senses for teaching in the upper primary grades. How many senses would we include in this model, and what would those senses be? In contrast to “The Five Traditional Human Senses”, the strongest alternative model is “The Nine Primary Human Senses”, consisting of:
- position (an easier word and concept than “proprioception”)
In conjunction with this model, it could be useful to teach our children “The Eleven Primary Animal Senses”, which consists of the above nine human senses, along with the magnetic sense of direction and the perception of electrical fields in salt water.
Although there is no consensus on a census of the senses, the 9-sense model is slowly gaining ground as an excellent model for educational purposes, and it is certainly a strong contender for inclusion in the curriculum of the upper primary grades. That said, there is also a reasonable 7-sense model, and a reasonable 11-sense model. A model with more than 20 senses is certainly possible, even though not particularly suitable for teaching in primary school. We should always remember that while the science models we teach our kids are helpful tools for learning, these models are usually a simplified approximation of reality, rather than a perfect and unassailable reflection of reality. Therefore we should not confuse our models with absolute truth.