Think You Only Have Five Senses? Think again

You have at least nine, and many scientists argue 21. So what are they?

People are under the misapprehension that we have only five senses – sight, hearing, touch, smell and taste. This probably originates from Aristotle’s treatise On the Soul, more commonly known as De Anima, where he states the five senses. 

This is wrong. Five is incorrect, by any measure you use. You have… well, it’s hotly debated. The argument stems from how a sense is defined. The scientists who study such things will give you different answers depending on their speciality and their personal definition.

For example, a sixth sense is when you are psychic or see dead people. (That was a joke.)

According to experts, humans have 9, 21 or lots more.

First, let’s look at the original five and what they sense, to see where Aristotle, and nearly everyone since, has gone wrong.

The five senses — a recap and a clue to where things start to go wrong

Sight

This is straightforward and generally agreed upon. Your eyes detect light reflecting off objects, and receptors in the retina convert it into signals that the brain interprets. But what about the ability to sense colour?

Hearing

Again, an easy one. Sound waves are funnelled into the ear and then hit the eardrum. Those signals are then sent to the brain, which decodes them.

Touch

This is where things get more complicated. Touch is generally believed to be the first sense humans develop. But it is harder to define than you might think. It’s more than simply something touching your skin, and is a good example of where things start to break down with the whole ‘five senses’ limit — more on this in a second.

Taste

It is generally agreed that we have five primary tastes: salty, sweet, sour, bitter, and umami. But does intensity count as an extra sense? Probably not. Does the ability to taste individual flavours count as a separate sense? Again, probably not, but if you can’t taste sweet things, do you have one less sense or just a poorer sense of taste? Although most would count taste as just one sense, the debate hints at the limitations of these classifications.

Smell

Similar to taste, most count smell as one overall sense. It is a receptor that can detect scents and that is then interpreted by your brain. Technically, however, your nose has hundreds of receptors.

Interesting fact: A 2014 study suggested that humans could distinguish more than one trillion different odours. However, subsequent researchers challenged the calculations, and the actual number remains unknown. Whatever the figure, that doesn’t mean we have a trillion different senses.

Generally, most agree that each of these five represents an individual sense. So, what else is there?

Senses that are not included in ‘the five’ but are definitely senses

I’ve touched on some debates in these matters. For example, taste or smell could be counted as one sense or many. The arguments are pretty weak, though, and stray into the pedantic. But others are less debatable, yet haven’t been included. Here are a few:

Proprioception — the sense of space

This is the sense of where your body and its extremities are. If you close your eyes, you can probably touch your nose (unless you’re really drunk). There are restaurants where you eat in the dark, and despite not being able to see, you can still manage to scoop up food and find your mouth.

It is because of proprioception that you can walk up or down stairs without having to look at each step. It allows you to catch a ball without looking at your hands. It is the sense that generally lets you know what angle your joints are at, where your left hand is, and so on.

If you have poor proprioception, you will probably be seen as a bit clumsy or uncoordinated.

Thermoception — the sense of temperature

You can sense heat and cold. This is different from touching a hot kettle. It is a sense that works with no tactile sensations whatsoever. For example, thermoception happens when you are in front of a bonfire on a cold night. Or when you arrive in a tropical country and step out of the air-conditioned airport and are hit with a wave of heat.

There is some debate as to whether being able to detect the cold is a distinct sense or if just noting the temperature counts as one overall sense.

This is something that is frequently unconscious, and the body reacts accordingly with shivering or sweating.

Equilibrioception — the sense of balance

Equilibrioception is fairly self-explanatory. There are receptors in the inner ear that have nothing to do with hearing. They are a distinct, complicated structure with separate paths to the brain. Again, balance is definitely a sense, but it is not one of the five. However, it is essential for us. For example, it helps us walk upright. 

As an aside, many people get queasy when trying VR. This is down to senses such as sight and sound not matching with others, like proprioception and equilibrioception. It results in confusion in the brain and a vague feeling of illness.

Nociception — the sense of pain

This is often included as part of touch but is actually separate.

Logically, if you are cut or struck, then you feel it as a result of the contact. But the pain doesn’t always arrive at the precise moment of impact. Also, the pain can linger for days, weeks or months, without further contact. Things like ripped ligaments, broken bones, bruises, and strains are sensed, but there is no new contact with a touch receptor.

What about other types of internal pain? A swollen appendix is a different type of discomfort compared to trapped gas. You can feel when something is wrong inside you, whether it’s indigestion or even the result of spicy food.

Then there are headaches and migraines.

There are people born with Congenital Insensitivity to Pain (CIP) who have no sense of pain. They lack nociception, and it is a dangerous sense to be without.

Other things that may or may not count as a sense

Some scientists include physiological senses that are essential, such as hunger and thirst. It doesn’t count as Nociception, as there is (usually) no pain. When food and hunger are sated, you know it. And what about interoception, the sense of needing the toilet or feeling full?

We can even sense things like blood pressure and external pressure, such as when you’re in an aeroplane. Staying with the aircraft example, when taking off, we can also feel gravity and forward acceleration.

Some argue that we should include muscles. We can feel muscles and tendons being used and stretched.

How about oxygen and carbon dioxide? If you hold your breath, you know about it.

Fine. So, how should senses be counted, and is there a consensus?

That’s the problem. Not everyone agrees, as shown by the multiple rhetorical questions above.

Most neurologists count nine as a minimum. These include the original five and the four mentioned above: proprioception, thermoception, equilibrioception, and nociception.

However, most scientists would include additional senses, such as hunger, blood pressure and others mentioned above. Including these would bring the total to 21 commonly agreed-upon senses. If you want a fuller list and others that are debated, you can see them here.

Sense and perception are hard to pin down. Not only are their definitions open to interpretation, but they also often overlap and can influence each other.

You experience a variety of sensory data all the time. Not only do they interact and overlap, but so do many of the fields of science that study them, including neurobiology, cognitive science and cognitive psychology.

So the answer could be nine. Or twenty-one. Or considerably more, depending on whom you ask. One thing is certain, though: Aristotle’s five were wrong.