How shall we determine when consciousness exists? What are its tests?
The response of the mind to stimuli, made evident by the body’s reaction, gives the proof of consciousness in man or lower animal.
But what do we mean by a stimulus?
Light stimulates me to close my eyes when first entering its glare from a dark room, or to open them when it plays upon my eyelids as I sleep and the morning sun reaches me. It is a stimulus from without.
The fear-thought, which makes my body tremble, my pupils grow wide, and whitens my cheeks, is a stimulus from within.
An unexpected shot in the woods near-by, which changes the whole trend of my thinking and startles me into investigating its cause, is a stimulus from without causing a change within.
A stimulus, then, is anything within or without the body that arouses awareness; and this is usually evidenced by some physical change, however slight—perhaps only by dilated pupils or an expression of relief. When we see the reaction of the body to the stimulus we know there is consciousness. On the other hand, we cannot say that consciousness is always absent when the usual response does not occur; for there may be injury to organs accounting for the lack of visible reaction, while the mind itself may respond. But with due care, in even such cases, some external symptoms of response can usually be found if consciousness exists.
We have already realized how complex, intricate, and changing is fully developed consciousness.
The Unconscious
But the mind of man knows two distinct conditions of activity—the conscious and the unconscious. Mind is not always wide awake. We recognize what we call the conscious mind as the ruling force in our lives. But how many things I do without conscious attention; how often I find myself deep in an unexplainable mood; how the fragrance of a flower will sometimes turn the tide of a day for me and make me square my shoulders and go at my task with renewed vigor; or a casual glimpse of a face in the street turn my attention away from my errand and settle my mind into a brown study. Usually I am alert enough to control these errant reactions, but I am keenly aware of their demands upon my mind, and frequently it is only with conscious effort that I am kept upon my way unswerved by them, though not unmoved.