awareness of uncomplicated existence. In another moment or two it is a
consciousness of a day's work or pleasure ahead, the necessity of
rising, dressing, planning the day, the alert reaction of pleasure or
displeasure to what it is to bring, the effort to recall the dreams of
sleep--the complicated consciousness of the mature man or woman. But I
started the day with a mental condition close to pure sensation, a vague
feeling of something different than what was just before.

Or this bare consciousness may come in the moment of acute shock, when
the sense of suffering, quite disconnected from its cause, pervades my
entire being; or at the second when I am first "coming back" after a
faint, or at the first stepping out from an anesthetic. In these
experiences most of us can recall a very simple mental content, and can
prove to our own satisfaction that there is such a thing as mere
awareness, a consciousness probably close akin to that of the lower
levels of animal life, or to that of the newborn babe when he first
opens his eyes to life.

_Consciousness_, then, in its elements, is the simplest mental reaction
to what the senses bring.

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
su