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 still developed in the simplest bit of living matter.  To learn their significance, therefore, we have turned to the study of the simplest form of matter in which these fundamental properties are manifested.  This led us at once to the study of the so-called protoplasm, for protoplasm is the simplest known form of matter that is alive.  Protoplasm itself at first seemed to be a homogeneous body, and was looked upon as a chemical compound of high complexity.  If this were true its properties would depend upon its composition and would be explained by the action of chemical forces.  Such a conception would have quickly solved the problem, for it would reduce living properties to chemical powers.  But the conception proved to be delusive.  Protoplasm, at least the simplest form known to possess the fundamental life properties, soon showed itself to be no chemical compound, but a machine of wonderful intricacy.