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The Greek philosopher Aristotle used sensory experiences and body parts to propose that humans have five senses. And you’ve probably heard of them before: Sight, smell, touch, taste, and sound.

But almost as soon as he proposed them, people noticed things that didn’t fit the bill. Like when you feel hungry or dizzy, do you sense that through one of the five senses or is that a separate sense? Scientists have debated how many senses you have for over two thousand years because it all comes down to what we consider a sense.

Scientists classify senses by distinguishing between sensation and perception. Sensation is all physical. It’s just the body detecting a piece of sensory information, what scientists call a stimulus.

And perception is organizing and interpreting that stimulus. So a perception is what we’re consciously aware of. For example, if you touch a cat, your skin can sense its fur, and as this information gets processed by the brain, you also perceive its softness.

So basically sensation is all about the body, perception is all about the brain. Neurons sense with tiny receptors that translate and transfer the electrical signal generated from a physical stimulus, like touching a cat. These signals then get organized and interpreted in different parts of our brains, leading to our conscious awareness and understanding of the stimulus like the softness of the cat’s fur.

And a lot of the time, when people talk about senses, this is what they really mean: perception; consciously experiencing or feeling something. So the number of senses that you have depends on which process, sensation or perception, you’re taking into consideration. If you think about the sensory stimuli themselves, there would only be three senses.

First, Light stimuli translates to vision, second, chemical stimuli is taste and smell, and, third mechanical stimuli account for touch and hearing. Then you have Aristotle’s original five senses, which were based on perceptions caused by external stimuli that were detected by visible sensory organs, like your eyes or nose. But there are definitely things we can sense that don’t have visible sensory organs attached to them.

For example, you can sense hot and cold, respectively, with your skin, but that is a different thing than touch. And we have a sense of balance, through the vestibular system in your inner ear, which is a detection of motion and spatial orientation. So a definition requiring external stimuli starts with these original five and adds temperature and balance, for seven total senses.

But we can also feel things that aren’t coming from the outside, like pain, which is another sense. And of course, the source of pain can be from something external, like a, a bump or a cut, but sometimes it’s a stomach ache or headache with no external cause. There’s also proprioception, which lets you sense movement and where your body is in space.

And that lets you judge things like how flexed or extended your limbs are while doing things like walking. If you close your eyes and let someone move your body around like a doll, you will be able to describe exactly how you are positioned without looking, because we can sense the angles of our joints and the length of our muscles. So seven external senses plus pain and proprioception brings us to nine total.

And nine is the favorite answer for a lot of neuroscientists. But some people add a general category for internal states like hunger, thirst, or needing to use the bathroom. So that’s 10 senses, I guess.

But what about things that we can detect but we’re not consciously aware of…. Are those senses? Our bodies have receptors for things like the acidity of our cerebrospinal fluid and our glucose levels, but we aren’t ever aware of having slightly acidic or basic spinal fluid.

You can also break some of the other senses into more specific divisions. So the category of an internal state could be broken into each state individually: hunger and thirst would be two different senses, and even needing to go to the bathroom would be separated into two senses. But also we can detect the intensity of light and color independently of each other, so maybe vision is actually two senses.

So if we think about senses as stimuli our body can detect, now we’re somewhere in the low 20s. But we can go higher. We could think about all the specialized and unique kinds of receptors as different senses.

Vision isn’t just broken down into brightness and color anymore. Color is subdivided into different wavelengths corresponding to red, green, and blue. Each color has a specific kind of receptor specialized for responding to its wavelength.

Taste is now salty, sweet, bitter, sour, and umami. Now we’re up to 33 senses. At least!

Cause that’s not counting the more than 400 different types of smell receptors, or each individual auditory receptor that corresponds with a different pitch. So, now we’ve all lost track, but I think we’re in the thousands of senses. And that’s just in us humans!

Other animals, like migratory birds, can navigate using the magnetic field of the Earth. But ultimately, this question is not how many senses you have but what a sense is. Which comes down to perception or how you make sense of the world surrounding you, which goes way beyond more than just the original five senses.

Thanks for watching this episode of SciShow Psych! If you would like to support this channel, go to Patreon.com/SciShowPsych to learn more. [♪ OUTRO].