II. Sensation as the Source
The mind works upon what exists. Our conscious mind is dominated by immediate perception, which may evoke memories of past perception, and together they enable us to project what the future might be. But our creative capacity to imagine a future is based to some degree on our experience and perception.
Therefore, it is worthwhile to consider the parts of the body that are not of the mind, but which provide input into the mind. They are separate, but the mind is heavily dependent upon the senses to have material to work upon.
It's also noted that the mind drives the body. The mind takes in information from the senses, translates it into thoughts, and the thoughts are often expressed through our speech and actions. The method by which the body works to perform actions is left to physicians to consider - psychology is not concerned about how we walk, but about the reason we would choose to walk to a given place.
Admittedly, the sensory systems are also external to the field of psychology - and it could be said that how we receive sensory data is outside the mind and should not be of much concern, but some basic information must be understood because sensation feeds the mind.
Sight
Sight is the primary sense of human beings: most of the information we receive about the world comes to us through our visual perception.
The sense of sight is the reception of data from the eyes. To be specific, we do not perceive objects, but the light that is transmitted or reflected from them. We receive hues, which are colors, shades, and intensities.
Contrary to common conception, the eye does not perceive shapes. The perception of shape is a mental process that defines areas by their hue. If we perceive a flower in the grass, it is because our mind recognizes that the area of white yellow within an area of green is something distinct, an object that we recognize and name through a mental process.
Likewise, the eyes do not perceive distance. They merely perceive a field of colors, the mind defines shapes that it believes to be objects, compares the appearance through the overlapping images perceived by each of our two eyes, and calculates its proximity.
An individual whose eyes are perfectly sound is not able to perceive objects if the areas of his brain that interprets them has been damaged. Likewise, a person with a perfectly functional brain can be fooled into perceiving distance where none exists, such as viewing a photograph or a painting, because it misinterprets the flat surface as having depth based on the color cues on two-dimensional plane.
Hearing
Hearing is second to vision in the amount of information we receive from the world - though when we gain information from listening to another person speak, it is arguably our hearing (though this, like viewing a painting, involves some level of artifice).
Sound is perceived through the ear, though the outer ear is merely a funnel that channels sound vibrations to the mechanisms of the inner ear. Just as vision perceives a field of colors, so does hearing perceive a cacophony of different sound vibrations.
And just as the eye does not perceive objects or distances, the ear does not perceive the sound of things. As we walk down a city street, we hear the noise of automobiles and people and the things that people are doing. A single sound, like the backfire of a vehicle, is not perceived as a separate thing by the ear, but by the mind that translates the sum total of all we hear into separate things, and identifies their probable origins.
The ears are capable of hearing only in a certain range. If a sound is too soft to create adequate vibration in the eardrums, it is not heard. If it is so loud that the vibrations exceed a certain threshold, all we perceive is pain. There are also limits to the pitch we can hear, and it is suggested (with evidence) that certain animals can hear tones that are too low or high in pitch for human beings to perceive.
Touch
The tactile sense is received through the human skin, which perceives pressure and temperature. Pressure can only be sensed when we are in physical contact with things, which limits our sense of touch perception as we cannot be in physical contact with all things at all times and for our physical safety we often avoid touching many of the things that we might.
The human skin varies in its sensitivity. The fingertips are highly sensitive to touch, but our shoulder blades are not - such that it takes greater pressure on some parts of the body for touch to be perceptible, and even then it may be perceived more vaguely such that finer textures are unnoticed.
Temperature, meanwhile, radiates from objects and can be felt at a distance. We do not need to touch a furnace to feel its heat because heat radiates from it. Nor do we need to touch ice to feel its heat because it draws heat away from objects that are close to it. It is much like sound, in that temperature deals with the movement of heat through air - and the closer we are to an object whose temperature differs from that of our skin, the more keenly we perceive its temperature.
And again, the sensation of touch from our skin is not interpreted by our skin - it is merely felt, all at once. It is in the mind that we recognize that an area in which we perceive temperature or pressure is caused by a specific thing.
Smell
Smell is perceived through the membranes in the nostrils. When we smell something, minute particles of the object are drawn into the nose and digested by the acidic mucous located there, and this information is transmitted to the brain.
And again, the notion that we smell specific things is the interpretation of the mind, not of the nose. Going back to the busy street, we smell many things all at once, and it is in our mind that we sort out these sensory perceptions to focus on a specific scent and to know its cause: we smell the bread in an oven of a bakery, but to know this is so requires us to isolate certain qualities of smell from others and understand their probable origins.
The more molecules we receive, the stronger a smell is perceived to be, though we are particularly sensitive to certain smells more than others - those that are associated to danger or opportunity come most quickly to our attention than others. But this too is a mental process, as we receive all smells and are attuned only to some.
Taste
The last of our senses is taste, which operates in the same way as smell (the digestion of molecules) but is limited to objects that are placed in the mouth, in contact with the tongue.
There is quite some argument over the division between these two senses - how much of what we perceive when we place something into our mouths is perceived by the tongue or the nostrils.
In general, it is assumed that the tongue has a limited number of receptive capabilities - that it tastes things as bitter, sweet, sour, or salty - but this is subject to a great deal of debate.
Somatic Sensations
(EN: The author does not mention somatic sensations, but I felt it worth mentioning in the interest of being comprehensive. Our ability to receive bodily sensations - we feel sickness or soreness - is not through the traditional senses of sight, hearing, touch, taste, and smell by which we receive input from the external world. And somatic sensations and memories of them play a large role in thinking.)
Extrasensory Perception
In the author's time, it was widely suspected that human beings have an ability to perceive things outside the realm of the five senses, but this is understood only in a vague and unscientific manner. And for that reason it is relegated to the questionable realm of parapsychology and mysticism - it is not yet the subject of any serious scientific inquiry and until it is better understood it should not intrude upon serious discourse.
Insofar is ESP is concerned, the scientific community regards it as a kind of mental activity: we interpret the sensory data in a way that we are unable to express in words. As such, when we perceive something through our intuition, it is not because we have received additional information but merely because we have not fully processed it to enable us to adequately describe it.
The unconscious or subconscious workings or the mind includes many thoughts and feelings we cannot express or fully comprehend. It is because something we have perceived through our normal senses has not been processed, not because we have received information from an additional sensory system.
The Nerves
The body is traversed by a system of nerves which is likened to a telegraph system. It is through these conduits that information collected by our various sensory systems are transmitted to the brain, as well as being used to transmit signals from the brain to the various parts of the body to direct their behavior.
Some behaviors are largely autonomous - the heart, lungs, digestive system, sweat glands, and other physical operations are controlled by parts of the brain to which the mind has no access. There are some philosophies that maintain we can gain control over these systems, but they are relegated to the questionable realm of parapsychology and will be considered no further.
Just as with wiring, nerve cables can be cut, and doing so renders parts of the body isolated from the mind - the mind cannot receive any sensation nor exert any control over a part of the body that has become disconnected from the central nervous system.
As with the blood vessels, the nerves vary in their thickness. The great spinal column is the central cable to which all others are wired (though it is argued that sensory and motor nerves of the head may be connected more or less directly to the brain), which branches out into smaller and smaller wires, down to the level of nerves that are like capillaries that permeate the skin.