ructure determines function, and function reacts
upon structure. This interaction goes on throughout life; cause and
effect interchange or play into each other's hands. The more power we
spend within limits the more power we have. This is another respect in
which life is utterly unmechanical. A machine does not grow stronger by
use as our muscles do; it does not store up or conserve the energy it
expends. The gun is weaker by every ball it hurls; not so the baseball
pitcher; he is made stronger up to the limit of his capacity for
strength.

It is plain enough that all living beings are machines in this
respect--they are kept going by the reactions between their interior and
their exterior; these reactions are either mechanical, as in flying,
swimming, walking, and involve gravitation, or they are chemical and
assimilative, as in breathing and eating. To that extent all living
things are machines--some force exterior to themselves must aid in
keeping them going; there is no spontaneous or uncaused movement in
them; and yet what a difference between a machine and a living thing!

True it is that a man cannot live and function without heat and oxygen,
nor long without food, and yet his relation to his medium and
environment is as radically different from that of the steam-engine as
it is possible to express. His driving-wheel, the heart, acts in
response to some stimulus as truly as does the piston of the engine, and
the principles involved in circulation are all mechanical; and yet the
main thing is not mechanical, but vital. Analyze the vital activities
into principles of mechanics and of chemistry, if you will, yet there is
something involved that is neither mechanical nor chemical, though it
may be that only the imagination can grasp it.

The type that prints the book is set up and again distributed by a
purely mechanical process, but that which the printed page signifies
involves something not mechanical. The mechanical and chemical
principles operative in men's bodies are all the same; the cell
structure is the same, and yet behold the difference between men in
size, in strength, in appearance, in temperament, in disposition, in
capacities! All the processes of respiration, circulation, and nutrition
in our bodies involve well-known mechanical principles, and the body is
accurately described as a machine; and yet if there were not something
in it that transcends mechanics and chemistry would you and I be here? A
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