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 machine is the same whether it is in action or repose, but when a body ceases to function, it is not the same.  It cannot be set going like a machine; the motor power has ceased to be.  But if the life of the body were no more than the sum of the reactions existing between the body and the medium in which it lives, this were not so.  A body lives as long as there is a proper renewal of the interior medium through exchanges with its environment.

Mechanical principles are operative in every part of the body—­in the heart, in the arteries, in the limbs, in the joints, in the bowels, in the muscles; and chemical principles are operative in the lungs, in the stomach, in the liver, in the kidneys; but to all these things do we not have to add something that is not mechanical or chemical to make the man, to make the plant?  A higher mechanics, a higher chemistry, if you prefer, a force, but a force differing in kind from the physical forces.

The forces of life are constructive forces, and work in a world of disintegrating or destructive forces which oppose them and which they overcome.  The mechanical and the chemical forces of dead matter are the enemies of the forces of life till life overcomes and uses them; as much so as gravity, fire, frost, water are man’s enemies till he has learned how to subdue and use them.