Most of us are normally quite unconscious of the limitations of our senses. We take for granted that our senses are completely dependable in providing us with an accurate idea of reality as it actually is. We assume that it is simply not possible for there to be any truer idea of reality than our own.
Remember the introductory speculations (“What if … ?”) just a short while back?
Whose perception of reality is more accurate — yours, or the electric eel’s? Or that of the bat, or the dog? Or are we all wrong? What is the right view of reality?
In fact, an accurate “right” view that corresponds exactly with physical reality is not possible, because of the way our senses work. All sense organs work by undergoing physical and chemical changes brought about by changes in the [internal or external] environment. In other words, with a change in the environment, there occurs a corresponding change in the appropriate sense organ, and it is this change in the sense organ that produces a “sensory signal”.
Certain tissues readily undergo changes with appropriate environmental stimuli, and these tissues are said to be “sensitive” to those environmental factors (like light, sound, chemical substances, heat, and so on) which they are specially adapted to respond to. Thus, the eye is sensitive to light, and the ear to sound; but shine a flashlight into the ear and the light will not be heard, because the tissues of the ear are insensitive to light.
Within the sensory function for which a sense organ is specially adapted, the physical and chemical changes caused by the appropriate stimuli must be reversible for the affected tissues to recover. If an environmental stimulus (light, noise, heat, chemical concentration, pressure, etc) exceeds critical intensities so as to cause irreversible change and tissue damage, the sense organ exposed to it will no longer be able to function. This imposes maximum levels of detectable intensity which cannot be exceeded. For example, from a possible temperature range of minus-273ºC to thousands of degrees or more, the body can effectively discern only within 33ºC ± 30º very (!) approximately. Outside this range, irreversible tissue damage will occur.
For any sense organ, the range of sensitivity is necessarily limited, partly because the organ is composed of destructible biological tissue, and partly for efficiency. The unprotected body can be stable only within a relatively narrow range of environmental variations, outside of which there will be tissue damage. It is within this narrow range conducive to bodily stability that each sensory function is optimally sensitive, and it is within the even narrower central range or “zone of comfort” that sensitivity is most acute.
There is another aspect of how each sensory function is limited. This other limitation is most conveniently illustrated using the eye as an example. The human eye is sensitive to light within the spectral range of approximately 375 - 750 nm wavelength, and is insensitive to light outside this bandwidth. (1 nm — a nanometre — is 10-9 of a metre, a millionth of 1 mm.)
This entire bandwidth of human visual sensitivity, spanning about 375 nm, is about one-twentieth of the diameter of a red blood cell: an infinitesimally narrow slit of an optical window through which to sample the electromagnetic spectrum with its range from near-zero to thousands of kilometres wavelength.
Certain kinds of snakes can detect infra-red radiation acutely enough for them to perceive images of warm-blooded animals, including prey, at night. 
Many insects, like bees, can see colours in the ultra-violet often reflected in flowers. Infra-red and ultra-violet are outside the range visible to the human eye, and we are blind to things many other creatures can see.
It is similar with the other senses. The vibration frequencies to which the human ear is sensitive are in the range of approximately 20 to 20,000 Hz (hertz, or cycles per second), and this band is just a narrow slit in the spectrum of sound and vibration. Elephants can hear low-pitched sounds that are inaudible to us. Many other animals can hear sounds that are pitched too high for us to hear. And the kinds of molecules to which our organs of smell and taste are sensitive are a tiny fraction of the kinds of molecules with which we may come into contact.
In addition to these limitations (in sensitivity to levels and ranges), there are many types of environmental factors (like radio waves, magnetic fields, inert gases, etc.) that our bodies are completely insensitive to, and cannot detect at all. It is true that there are ways to extend the detectable types of environmental factors, and their ranges, beyond normal human sensitivity by using artificial devices. However, artificial detection of such types of factors or extended ranges, unlike the case with the bodily senses, cannot be experienced directly, but only indirectly through an “interpreter” such as a meter or gauge, i.e., by inference.
There is a vast real difference between direct experience and inference. If you and another are each having a cup of tea, poured out at more or less the same time from the same pot, you can directly experience the temperature of the tea from your own cup while sipping it, but can only infer that the tea in the other cup is probably at a similar temperature, and be content with the other’s description or interpretation.
Our sense organs sample limited aspects of reality through very tiny slits appropriate to biological survival under naturally recurrent conditions, and within these slits, it is change (serial difference in space-time) in the environmental factors that the senses are best suited to detect. Even here, the rate of change has to be appropriate.
When the rate of change in these factors is too slow, the sense organs will either ignore them or fail to detect them at all, e.g., a continuous noise in the background, the sensation in your buttocks when you are seated, the taste in your mouth, a lingering odour in a room. (Even when staring at a static object, the eyes must make rapid, near-random flicking movements to produce the change necessary for focused visual perception. When the movements are inhibited, the vision quickly becomes blurred.) Change that occurs too quickly (e.g., the normal flashing that occurs with all fluorescent lights) is also imperceptible.
It should be clear by now that there are serious limitations to our senses, and that there are unimaginably many things we can neither perceive nor even detect. Can we still take for granted that our senses are completely dependable in providing us with an accurate idea of reality as it actually is? Can we even believe that our idea of reality closely resembles the way all things actually are? Is it possible that what we cannot perceive is far more than the little that we can?
