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. When we realize that nothing that has ever happened in our experience is forgotten; that nothing once in consciousness altogether drops out, but is stored away waiting to be used some day--waiting for a voice from the conscious world to recall it from oblivion--then we grasp the fact that the quality of present thought or reaction is largely determined by the sum of all past thinking and acting. Just as my body is the result of the heritage of many ancestors plus the food I give it and the use to which I subject it, so my mind's capacity is determined by my inheritance plus the mental food I give it, plus everything to which I have subjected it since the day I was born. For it forgets absolutely nothing. "That is not true," you say, "for I have tried desperately to remember certain incidents, certain lessons learned--and they are _gone_. Moreover, I cannot remember what happened back there in my babyhood."