The External Senses in a Human Being

As a human being, one experiences life as sensate experiences (the outer world, for convenience) and as mental experiences (the inner world, for convenience). Certain experiences are sensate but within the body (for example: positional awareness, a pain in the abdomen, fluttering of the heart, the rush of blood); these experiences can be considered as internal experiences.

In this article, I will make a few comments on the external sense organs. Through these organs, we get information about the outside world. We have the eyes, the ears, the nose, the tongue, and the skin which provide us various kinds of information.

The eyes are sensitive to EM radiation within a certain bandwidth. The ears are sensitive to matter waves within a certain bandwidth. The nose and the tongue have receptors which respond to certain chemicals. The skin is sensitive to tactile stimulation and to temperature.

The eyes are the most powerful of the sense organs because of many reasons:

• Their informational bandwidth capacity ("a picture is worth a thousand words", etc.)
• Their ability to provide information at vast distances ("the stars")
• Their speed of the signalling (light is the fastest of all known signalling systems)
• The perspective switching capacity ("avert the sight", "get a different view", "close your eyes", etc.). This switching capacity is absent in other senses. The ears can be plugged to all input, but cannot be operated to filter out certain inputs (Noise Canceling headphones facilitate that!)
  

We are primarily visual creatures. Our dreams and hallucinations (i.e. our self-generated imaginary sensual experiences) are primarily visual phenomena. The very word "imagination" has the word "image" as its root.

The ears are next, since they can also receive information at a distance. I find that our emotions are more deeply linked with aural input than with visual input, though I am unable to delve deeper into it at the moment.

Music can easily create a mood. And our thinking and intellection is closer to being aural than to any other sense. We "hear" our thoughts in our minds. It is also interesting to note that aural silence (for example in a deep forest, or in a meditation room) can lead to a temporary inner silence which is very rejuvenating. "Silence" is frequently talked about in spiritual circles as a very significant means and as a significant end (it is considered a peaceful state). Visual silence (an oxymoron, but I mean: closed eyes) is not that powerfully capable of inducing an inner state of peace and stillness, even temporarily, unless it be accompanied by aural silence.

The nose and the tongue are both chemical senses (though the tongue is very sensitive as a tactile organ as well). Of the two, the nose is much more sensitive than the tongue and the sense of smell much more significant than that of taste. The sense of smell can operate at a distance, through a gaseous medium, whereas the sense of taste cannot. These senses are the most closely tied to food and nutrition. Due to this important function, the strongest likes and dislikes are in these two senses. Blind nature has equipped us with an in-built discriminatory system so that we don't end up poisoning ourselves, and that we know what is nourishing. This discrimination is not perfect and flawless, but it works pretty well as an approximation. Food which smells and tastes "good" is usually nourishing and energy-rich.

"Sweetness" and "bitterness" is our response to a foodstuff. It is similar to a painting being beautiful or ugly. The object itself is not sweet or bitter or beautiful or ugly. Our chemical receptors, when they interact with a food which has a high glycemic index, send green signals, so to speak, to our brains. These green signals are what we experience as "sweetness" in the foodstuff.

More details, for a layperson, here. The relevant excerpt is:

When sugars come into contact with the tongue, they bind to a sweet taste receptor proteins that trigger a cascade of biochemical sweetness signals to the brain

These two are the most primitive senses, and a human infant is extremely sensitive to chemical inputs through these sense organs right from its birth. Some biologists claim, rightly I think, that the imprinting on an infant of its mother happens through the sense of smell.

The skin responds to tactile and thermal inputs. This sense organ is the largest in its surface area in human beings and comfort is therefore primarily linked with comfort for the skin. The other external sense organs are located in the facial region. "Sensuality", and especially sexual pleasure, is most closely associated with tactile pleasure (though in sex, all sense organs are involved to some extent). The skin has different concentrations of nerve endings, and different kinds of nerve endings, in various parts of the body and hence tactile stimulation of those different parts lead to sensations varying in quality and amplitude.