We often use the term 6th sense to describe extra sensory perception. This is because we’ve been lead to believe that we have just 5 basic senses, sight, hearing, smell, taste and touch. These senses are the most obvious and correspond to physical sensory organs, parts of the human body that act as a gateway from the physical world to the realm of the mind, allowing us to perceive the world around us.
It’s likely that this “five senses” model is what you’ve been taught in school, and it has been the standard for quite a while, even since ancient times. Cursor Mundi describes these five outward senses centuries ago, though Aristotle believed there were just four, considering taste to be a subset of touch.
Hering, sight, smelling and fele, cheuing er wittes five,
All sal be tint er sal pas, quen þe hert sal riue.Cursor Mundi, lines 17017–17020
The Roman author Aulus Gellius stated “Nature has given five senses to living beings, sight, hearing, taste, touch, and smell, called αισθητεισ by the Greeks”. In Hindu literature, the Katha Upanishad presents the five senses as five horses of a chariot, with the Atman (soul) as the charioteer.
Know that the Atman is the rider in the chariot, and the body is the chariot,
Know that the Buddhi (intelligence, ability to reason) is the charioteer, and Manas (mind) is the reins.
The senses are called the horses, the objects of the senses are their paths,
Formed out of the union of the Atman, the senses and the mind, him they call the “enjoyer”.Katha Upanishad, 1.3.3-1.3.4
Gérard de Lairesse’s Allegory of the Five Senses depicts the five senses represented by the individual people in his painting. Have a look and see if you can spot all five.
But even then, many great thinkers and philosophers believed that there was more than met the eye. The Buddhist Ayatana acknowledged the five senses but considered the mind to be a sense organ as well, with the ability to sense mental objects like feelings, perceptions and impressions. In Theaetetus, Plato quotes Socrates as saying that many more unnamed senses exist, including sight, smell, hearing, temperature, pain, pleasure and fear.
In fact, the more we study the senses, the more there appear to be. For example, where does your sense of balance come from? How do you know where your limbs are even when your eyes are closed? What makes you feel full after a heavy meal and what makes you thirsty? So with a few basic questions, the five senses theory is easily proven to be insufficient. Now let’s have a look at another table of potential senses.
|• 2000 or more |
|• light touch||✓|
|• rotational acceleration||✓|
|• linear acceleration||✓|
|• proprioception – joint position||✓||✓|
|• muscle stretch – Golgi tendon organs||✓||✓|
|• muscle stretch – muscle spindles||✓||✓|
|• blood pressure||✓||✓|
|• arterial blood pressure||✓|
|• central venous blood pressure||✓|
|• heat blood temperature||✓|
|• blood oxygen content||✓||✓|
|• cerebrospinal fluid pH||✓||✓|
|• plasma osmotic pressure – thirst||✓||✓|
|• artery vein blood glucose – hunger||✓||✓|
|• lung inflation||✓||✓|
|• bladder stretch||✓|
|• full stomach||✓|
So it seems there’s a lot more to it. Our sensory organs respond to an extremely large variety of stimuli in order to provide us with a massive amount of data about our bodies and surroundings. These sensory organs are composed of sensory receptor cells like mechanoreceptors, photoreceptors, thermoreceptors, and chemoreceptors, which provide both exteroception and interoception. While some of these are just common sense, the others are much more subtle and we may not consciously feel them. I mean, when was the last time you sensed the pH value of your cerebrospinal fluid.
And of course, there could be many more senses, for example our responsiveness to the circadian rhythm, or how we perceive attractiveness, or how we know that someone loves us, or how we sense the passing of time, or how some people have an amazing sense of direction. So until we answer all these questions, let’s just assume there are many more. It just makes sense.