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.
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