ost general mark of Mind is sensation or common feeling.
In technical language a sensation is defined to be the result of an
impression on an organism, producing some molecular change in its nerve
or life centres. It is the consequence of a contact with another
existence. Measured by its effects upon the individual the common law of
sensation is: Every impression, however slight, either adds to or takes
from the sum of the life-force of the system; in the former case it
produces a pleasurable, in the latter a painful sensation. The
exceptions to this rule, though many, are such in appearance only.[9-1]

In the human race the impression can often be made quite as forcibly by
a thought as by an act. "I am confident," says John Hunter, the
anatomist, "that I can fix my attention to any part, until I have a
sensation in that part." This is what is called the influence of the
mind upon the body. Its extent is much greater than used to be imagined,
and it has been a fertile source of religious delusions. Such sensations
are called subjective; those produced by external force, objective.

The immediate consequent of a sensation is _reflex action_, the object
of which is either to avoid pain or increase pleasure, in other words,
either to preserve or augment the individual life.

The molecular changes incident to a sensation leave permanent traces,
which are the physical bases of memory. One or several such remembered
sensations, evoked by a present sensation, combine with it to form an
Emotion. Characteristic of their origin is it that the emotions fall
naturally into a dual classification, in which the one involves
pleasurable or elevating, the other painful or depressing conditions.
Thus we have the pairs joy and grief, hope and fear, love and hate, etc.

The question of pleasure and pain is thus seen to be the primary one of
mental science. We must look to it to explain the meaning of sensation
as a common quality of organism. What is the significance of pleasure
and pain?

The question involves that of Life. Not to stray into foreign topics, it
may broadly be said that as all change resolves itself into motion,
and, as Helmholtz remarks, all science merges itself into mechanics, we
should commence by asking what vital motions these sensations stand for
or correspond to.

Every organism, and each of its parts, is the resultant of innumerable
motions, a composition of forces. As such, each obeys the first law of
motion