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into some form of active energy, and, like other machines, its power
ceases when the machine is broken.

With this understanding the problem clearly resolved itself into two
separate ones. The first was to determine to what extent known physical
and chemical laws and forces are adequate to an explanation of the
various phenomena of life. The second was to determine whether there are
any known forces which can furnish a natural explanation of the origin
of the living machine. Manifestly, if the first of these problems is
insolvable, the second is insolvable also.

In the study of the first problem we have reached the general conclusion
that the secondary phenomena of life are readily explained by the
application of physical and chemical forces acting in the living
machine. These secondary phenomena include such processes as the
digestion and absorption of food, circulation, respiration, excretion,
bodily motion, etc. Nervous phenomena also doubtless come under this
head, at least so far as concerns nervous force. We have been obliged,
however, to exclude from this correlation the mental phenomena. Mental
phenomena can not as yet be measured, and have not yet been shown to be
correlated with physical energy. In other words, it has not yet been
proved that mental force is energy at all; and if it is not energy, then
of course it can not be included in the laws which govern the physical
energy of the universe. Although a close relation exists between
physical changes in the brain cells and mental phenomena, no further
connection has yet been drawn between mental power and physical force.
All other secondary phenomena, however, are intelligently explained by
the action of natural forces in the machinery of the living organism.

While we have thus found that the secondary phenomena of life are
intelligible as the result of the structure of the machine, certain
other fundamental phenomena have been constantly forcing themselves upon
our attention as a _foundation_ of these secondary activities. The power
of contraction, the power of causing certain kinds of chemical change to
occur which result in metabolism, the property of sensibility, the
property of reproduction--these are fundamental to all living activity,
and are, after all, the real phenomena which we wish to explain. But
these are not peculiar to the complicated machines. We can discard all
the apparent machinery of the animal or plant and find these properties
s