Sensations.—­Pursuing this subject further, we next notice that it is possible to trace a connection between physical energy and sensations.  Sensations are excited by certain external forms of motion.  The living machine has, for example, one piece of apparatus capable of being affected by rapidly vibrating waves of air.  This bit of the machine we call the ear.  It is made of parts delicately adjusted, so that vibrating waves of air set them in motion, and their motion starts a nervous stimulus travelling along the auditory nerve.  As a result this apparatus will be set in motion, and an impulse sent along the auditory nerve whenever that external type of motion which we call sound strikes the ear.  In other words, the ear is a piece of apparatus for changing air vibrations into nervous stimulation, and is therefore a machine.  Apparently the material in the ear is like a bit of gunpowder, capable of being exploded by certain kinds of external excitation; but neither the gunpowder nor the material in the ear develops any energy other than that in it at the outset.  In the same way the optic nerve has, at its end, a bit of mechanism readily excited by light vibrations of the ether, and hence the optic nerve will always be excited when ether vibrations chance to have an opportunity of setting the optic machinery in motion.  And so on with the other senses.  Each sensory nerve has, at its end, a bit of machinery designed for the transformation of certain kinds of external energy into nervous energy, just as a dynamo is a machine for transforming motion into electricity.  If the machine is broken, the external force has no longer any power of acting upon it, and the individual becomes deaf or blind.

Mental Phenomena.—­Thus far in our analysis we need not hesitate in recognizing a correlation between physical and nervous energy.  Even though nervous energy is very subtle and only affects our instruments of measurements under exceptional conditions, the fact that nervous forces are excited by physical forces, and are themselves directly measurable, indicates that they are correlated with physical forces.  Up to this point, then, we may confidently say that the nervous system is part of the machine.

But when we turn to the more obscure parts of the nervous phenomena, those which we commonly call mental, we find ourselves obliged to stop abruptly.  We may trace the external force to the sensory organ, we may trace this force into a nervous stimulus, and may follow this stimulus to the brain as a wave motion, and therefore as a form of physical energy.  But there we must stop.  We have no idea of how the nervous impulse is converted into a sensation.  The mental side of the sensation appears to stand in a category by itself, and we can not look upon it as a form of energy.  It is true that many brave attempts have been made to associate the two.  Sensations can be measured as to intensity, and the intensity of a sensation is to a certain extent dependent