Guest post by John M. Henshaw
Just how many senses does a human being have? If you Google this question you will find, as with just about anything else you might care to Google, a variety of answers. Some say we have seven senses, while others put the total at nine, ten, or twelve. What’s the right answer? It all depends on how you define things.
Let’s first observe that all of the numbers in the paragraph above are greater than five. It doesn’t take much reflection to figure out that humans possess more than the five “classical” senses of sight, hearing, taste, smell, and touch.
The idea of five classical senses dates back at least to Aristotle, himself a rather classy guy. In De Anima (Of the Soul) he argues that, for every sense, there is a sense organ. So far, he’s on reasonably solid ground. It’s when he goes on to say that there can be no sixth sense, because there are only five sense organs, that he gets himself into trouble.
It doesn’t take much reflection to figure out that humans possess more than the five “classical” senses of sight, hearing, taste, smell, and touch.
Because when you start counting sense organs, you get to six right away: the eyes, ears, nose, tongue, skin, and the vestibular system. Our understanding of the vestibular system’s role as a sense organ dates only to the early 1800s, more than two millennia after Aristotle. We now know that the vestibular system, located in the inner ear, is an integral part of how we balance ourselves, but it also plays a critical role in vision, allowing us to keep our two eyes focused on things even while our heads are moving about.
So, six sense organs are quickly identified, but that doesn’t get us to nine, ten, or twelve senses. Let’s tweak Aristotle’s definition of what a sense is just a bit. Instead of a sense organ, each separate sense really only requires a different kind of sensory receptor. In the skin alone, there are at least four different kinds of sensory receptors: those for touch, temperature, pain, and proprioception (or body awareness). A sensory receptor is a specialized cell that sends electrical signals to the brain in response to the type of stimuli the cell is optimized for. The rods and cones in the retina are sensory receptors. They send signals when stimulated by light of various wavelengths and intensities. The skin is brimming over with sensory receptors optimized not only for touch, but for other things as well, such as hot and cold. There are at least six different kinds of temperature receptors, each optimized for a different temperature range.
If we have six different kinds of temperature receptors, does that mean that our ability to sense hot and cold is really six different senses, and not just one? I suppose you could argue that, but what would be the point? Consider human vision.
Human eyes contain four different kinds of sensory receptors: three types of cones (optimized for long, medium, and short wavelength light) and rods (optimized for low light conditions). Thus equipped, human beings can “see.” We have “vision.” But that’s only the beginning of the story. Human vision entails the ability to distinguish light from dark. For some primitive creatures, this is as far as their vision takes them. We humans can tell light from dark, we can distinguish images, we can see in color (as a result of having three types of cones), and, having two eyes, we possess stereovision. So just how many senses do our eyes afford us? One? Two? Three? Four? The conventional wisdom says we’ve got eyes, we can see, and that’s one sense. Good enough for me.
The point of all this is that it is harder than it might first appear to put a definitive figure on the total number of senses that humans possess. At some point, it becomes just a bit arbitrary. So here’s my list of nine human senses, which may be a little longer, or shorter, than yours:
- Proprioception (body awareness)
John M. Henshaw is Harry H. Rogers Professor and Chair of the Department of Mechanical Engineering at The University of Tulsa. His books, A Tour of the Senses: How Your Brain Interprets the World, and Does Measurement Measure Up? How Numbers Reveal and Conceal the Truth, are published by JHU Press.