e "circle of eternal change," is constantly
transforming the inorganic into the organic, and after using it,
returning it again to the realm of the inorganic. Whatever it does not
assimilate of that which it has taken in simply as a stimulant, and
whatever has become dead, it separates from itself and rejects. The
organism is in perfect health when it accomplishes this double task of
organizing and disorganizing. On the comprehension of this single fact
all laws of physical health or of hygiene are based. This idea of the
essence of life is expressed by Goethe in his Faust, where he sees the
golden buckets perpetually rising and sinking.[13] When the equilibrium
of the upward and downward motion is disturbed, we have disease. When
the motion ceases we have death, in which the whole organism becomes
inorganic, and the "dust returns to dust."

Sec. 54. It follows from this that not only in the organism as a whole, but
in every organ, and every part of every organ, this restless change of
the inorganic to the organic is going on. Every cell has its own
history, and this history is only the same as that of the whole of which
it forms a part. Activity is then not inimical to the organism, but is
the appointed means by which the progressive and retrogressive
metamorphoses must be carried out. In order that the process may go on
harmoniously, or, in other words, that the body may be healthy, the
whole organism, and every part of it in its own way, must have its
period of productive activity and then also its period of rest in which
it finds renewal of strength for another period of activity. Thus we
have waking and sleep, inspiration and expiration of air. Periodicity is
the law of life. When we understand the relative antagonism (their stage
of tension) of the different organs, and their cycles of activity, we
shall hold the secret of the constant self-renewal of life. This thought
finds expression in the old fairy stories of "The Search after the
Fountain of Youth." And the figure of the fountain, with its rising and
falling waters, doubtless finds its origin in the dim comprehension of
the endless double movement, or periodicity of life.

Sec. 55. When to any organ, or to the whole organism, not sufficient time
is allowed for it to withdraw into itself and to repair waste, we are
conscious of fatigue. While the other organs all rest, however, one
special organ may, as if separated from them, sustain a long-continued
effort