[This is Part 1 of a 3-part essay. Parts 2 and 3 will appear in a few days.]
Can you answer the following questions?
1) How many human senses are there? What are they?
2) How many planets do we have in our solar system, and what are their names?
3) Name all the continents in the world, and point out each one on a globe.
Before we discuss the answers to these questions, let’s first think about the questions themselves. What exactly are we doing when we ask questions such as these? Each of these questions is based on a classification system. So when we ask questions such as these, we are testing how well the responder has mastered the details of the classification system. With these 3 questions, we assume the existence of three distinct classes – senses, planets, and continents. We also assume that for any “thing” that we can name, we can categorize that thing as either being in the class or not in the class. For example, a turtle is not in any of these three classes – it is not a sense, a planet, or a continent. Jupiter fits into one of our three classes, because it is a planet – but it is not a sense or a continent. Africa is a member of another class, because it is continent – but it is not a sense or a planet. And taste is a sense – but it is not a planet or a continent.
As we will see in later essays, there is more than one way to structure a classification system. However, the three classification systems above – senses, planets, and continents – share a similar structure. In each case, the class contains fewer than 10 items, so we can easily memorize the entire list of “things” in the class. Furthermore, we can easily discuss the class in isolation from all other classes. For example, when we first teach children about planets, we don’t usually discuss asteroids and comets – that comes later in their education. When we first teach children about continents, we don’t expect them to simultaneously learn the locations of all the countries in the world. But later on, when the kids start to learn the locations of various countries, the most important location identifier is the continent in which the country is located.
Did you find that these three questions all look familiar? Basic classification models such as these are a fundamental part of what we teach our kids in school. Once our kids start kindergarten, classification models are a key component of the curriculum from that point on. Furthermore, for each of the above questions – and for other questions of the same type – we expect our students to provide the one acceptable “right” answer. And yet in each case, as we’ll see shortly, it is debatable as to whether the “right” answer is the only reasonable answer.
Human Senses
Question: How many human senses are there? What are they?
The standard answer, of course, is that there are 5 human senses – sight, hearing, taste, smell, and touch. This answer is so embedded in our culture that we have a common phrase – “sixth sense” – that refers to a person’s ability to perceive things that most other people cannot perceive. And yet the traditional answer of 5 human senses is not the only reasonable answer to the question. Imagine that there is someone — say, a visitor from another planet – who is unfamiliar with the usual answer to the question. Now imagine that this visitor is given the task of itemizing all of the human senses. How many senses would be included in the visitor’s list, and what would those senses be?
Because the visitor is unfamiliar with the traditional answer, he must use a set of criteria – rules of thumb – to sort through the possible answers. The criteria are derived from the definition of the class. So the first step is to define the word “sense”. What exactly is a “human sense”?
It turns out that the wording you use to define the phrase “human sense” can have a big impact on how many “things” can be included in the class. However, I would define “human sense” as “the ability of a human body to detect external matter, external forces, or external sources of energy”. This detection is enabled by “sense organs” within the body. Therefore a more complete definition would be “the ability of a human body to detect external matter, external forces, or external sources of energy, using sense organs within the body”.
Based on the above definition, anything that we would consider to be a human sense should meet the following criteria:
1) The sense enables us to detect external matter (that is, matter that is not part of our body) or an external force or source of energy.
2) It is possible to explain the nature of the matter, force, or energy that is detected by this specific sense.
3) It is possible to identify the specific part of the body that serves as the sense organ for this sense.
4) It is possible to explain how the sense organ detects the external matter, force, or energy.
The five traditional senses all meet the above criteria, as summarized below:
Sight – detects light. The sense organs are the eyes. Receptors in the retina of the eye respond to certain wavelengths of light. There are 4 types of receptors in the retina, each responding to a specific range of wavelengths. This allows us to distinguish among the wavelengths, and we perceive the various blends of wavelengths as different colors. The optic nerve conveys the data from the eyes to the brain. The brain processes the data to construct our sense of vision.
Hearing – detects sound waves in the air. The sense organs are the ears. Sound waves in the air trigger vibrations in the bones of the inner ear, which are detected by “hair cells” that send the data to the brain via the auditory nerve. The data includes information such as the mix of frequencies (pitches) in the sound and the loudness of the sound.
Taste – detects certain kinds of molecules that come in contact with the tongue and other tissues inside the mouth. The sense organ is therefore the mouth. The receptors are located in “taste buds” scattered along the surface of the tongue and on certain other tissues such as the soft palate. There are at least 5 kinds of taste buds, capable of detecting sweet, salty, sour, bitter, and savory molecular compounds. As with the other senses, the data is transmitted to the brain via the nervous system, and the brain processes the data to result in our perception of taste. (However, when we eat food, the brain blends taste data and smell data to create our perception of taste.)
Smell – detects certain kinds of airborne molecules that enter the nose. The sense organ is therefore the nose. The receptors are located in the olfactory bulb within the nasal cavity. There are apparently hundreds of different kinds of olfactory receptors, providing us with the ability to detect and distinguish a wide range of odors. The data is conveyed to the brain via the olfactory nerve, and the brain processes the data to create our perception of smell.
Touch – detects objects which come in contact with our skin. The sense organ is therefore the skin. The receptors are located beneath the skin, and detect pressure upon the skin. The wide distribution of the receptors allows us to deduce various properties of the object touching us, such as its texture and size. As with the other senses, the data is transmitted to the brain via the nervous system, and the brain processes the data to produce our sense of touch. In addition to the pressure receptors, there are several other kinds of receptors in the skin, including hot, cold, and pain. However, these various sensations are traditionally lumped into the sense of touch.
Okay, so we know that our visitor from another planet will include at least 5 human senses in his list. But will he find any other human senses to add to his list?
The visitor – let’s call him Marvin – does indeed catalog some additional senses. The next sense on his list is the sense of balance, and this is what Marvin has to say about it:
Balance – detects the direction of the force of gravity, allowing humans to determine whether their bodies are exactly vertical or slightly tilted. Human have long bodies, yet they walk on just two feet, balancing their long bodies perpendicular to the ground. Their sense of balance is extremely helpful when performing such activities as standing or walking, allowing them to maintain their balance and not fall over. The sense organ is the located in the inner ear (the semicircular canals), but is quite distinct from the sense of hearing. Fluid in the inner ear reacts to gravity, allowing the organ to detect the direction of the gravitation force. The fluid also reacts to acceleration forces, such as movements of the head. This information is conveyed to the brain via the vestibular nerve.
Marvin also draws a distinction between the sense of touch and the sense of hot and cold. Therefore, in Martin’s list we see the following seventh sense:
Hot and Cold – detects heat and coldness. The sense organ is the skin. Receptors located in the skin react to increased or decreased heat energy. However, these are not the same receptors responsible for the sense of touch. The change in heat energy may be due to conduction – either from a hot or cold object touching the skin, or from hot or cold air or water touching the skin – or the change in heat energy may be due to a net gain or loss by means of thermal radiation, such as feeling the warmth of sunlight or the heat from a fire. As with the other senses, the data is transmitted to the brain via the nervous system, and the brain processes the data to produce the sense of hot or cold.
The upshot is that Marvin’s list itemizes 7 human senses:
1) Sight (vision)
2) Hearing
3) Taste
4) Smell
5) Touch
6) Balance
7) Hot and cold
On the other hand, some people who study human senses say that there are 9 human senses. The other two senses, not on Marvin’s list, are the sense of pain, and the kinesthetic sense. (The kinesthetic sense, also called proprioception, is the ability to sense the position in space of various parts of the body.) However, Marvin did not include these latter two senses in his list, because in his opinion they do not involve the detection of external objects, forces, or sources of energy.
So now we have 3 different lists of human senses – one that lists 5 senses, one that lists 7 senses, and one that lists 9 senses. Can we really say that one of these models is “correct” and that the other two are “wrong”? In fact, each of these three models has a certain value. So what should we teach our kids? Should we teach them all 3 models?
Personally, if I could choose which model to use for instruction in schools, I would choose the model of 7 human senses. The 9-sense model would be my second choice. The 5-sense model is the weakest, because it limits our thinking too much. Any yet, teaching any of these models to our kids – even the 5-sense model – provides a lot of value. Any one of these models provides a framework upon which our kids can attach knowledge that they acquire. Therefore any of these models facilitates additional learning.
I would definitely not recommend that we present all 3 models on an equal footing to our kids. We should present one model as the standard, especially to younger children. However, we should not hide the fact that other models exist. As the kids get a little bit older, and have already become quite familiar with the standard model, it can be quite beneficial to point out that other non-standard models exist. Discussing these alternative models with our kids can significantly stretch their imaginations, helping them to see things in a different way. This is good training for being able to “think outside the box” as they grow older.
As a side note, we might ask the question, “If the 5-sense model of human senses is the weakest model, then how did the model originate, and how did we get so attached to it?” In my opinion, the 5-sense model is not really a model of the senses, but a model of the externally visible sense organs. There are 5 such organs – eyes, ears, mouth, nose, and skin. I believe that the focus on externally visible sense organs is the reason for including only 5 senses in the traditional model – in a one-to-one correspondence with the 5 organs. We remain attached to this model, despite its weaknesses, because it is so deeply rooted in our culture. If, on the other hand, a science educator were to be given the task of creating a better model for use in the primary school curriculum, the educator would probably choose a model that includes more than 5 senses – perhaps 7 or 9.
Now, suppose that we were to extend the model to include not just human senses, but the senses of all vertebrate animals. Would this add any more senses to our list, or would the list remain unchanged?
The one strong candidate to add to our list is a magnetic sense of direction. Some animals, such as certain birds, have the ability to detect magnetic fields – in particular, the magnetic field of the earth. There is actually an organ in the animal’s body that detects the orientation of the earth’s magnetic field relative to the animal’s body – something like a magnetic compass. Animals that possess this organ have a built-in sense of direction that is not dependent upon landmarks or the position of the sun in the sky. Such an organ is especially useful to birds who migrate long distances over the ocean. Therefore, if our model is expanded to cover all vertebrate animals, then the 7-sense model should be expanded to include at least 8 senses.
As we look through the world of animals – and now let’s include invertebrates as well as vertebrates – we can see examples of senses that appear to be different than human senses. We see snakes sensing the world by flicking out their forked tongues. We see bats navigating by means of echo location. And we see insects using their antennae to gather information about the world around them. But upon closer examination, we see that all of these senses share certain characteristics with human senses. If we define the sense of the smell as the ability to detect various kinds of molecules in the air, then it doesn’t matter whether the animal uses a nose or some other organ to do so. If we define the sense of taste as the ability to detect various kinds of non-airborne molecules that come in direct contact with the sense organ, then it doesn’t matter whether the animal uses its mouth or some other organ to do so. If we define the sense of hot and cold as the ability to detect heat gain or loss – either by means of conduction or thermal radiation – then it doesn’t matter whether the animal uses its skin or some other organ to do so. Even the bat’s ability to navigate by echo location can be described as an extension to the sense of hearing.
So that concludes our examination into the first class on our list – human senses. What about the next class on our list – planets?
[To be continued... Part 2 of the essay will appear in a few days.]
Copyright © 2010 by R. Philip Bouchard. All rights reserved.
