the same, and
everywhere one and the same in her efficacy and power of action;
that is, nature's laws and ordinances, whereby all things come to
pass and change from one form to another, are everywhere and
always the same; so that there should be one and the same method
of understanding the nature of all things whatsoever, namely,
through nature's universal laws and rules. Thus the passions of
hatred, anger, envy, and so on, considered in themselves, follow
from this same necessity and efficacy of nature; they answer to
certain definite causes, through which they are understood, and
possess certain properties as worthy of being known as the
properties of anything else, whereof the contemplation in itself
affords us delight. I shall, therefore, treat of the nature and
strength of the emotions according to the same method, as I
employed heretofore in my investigations concerning God and the
mind. I shall consider human actions and desires in exactly the
same manner, as though I were concerned with lines, planes, and
solids.

DEFINITIONS

I. By an adequate cause, I mean a cause through which its effect
can be clearly and distinctly perceived. By an inadequate or
partial cause, I mean a cause through which, by itself, its
effect cannot be understood.

II. I say that we act when anything takes place, either within
us or externally to us, whereof we are the adequate cause; that
is (by the foregoing definition) when through our nature
something takes place within us or externally to us, which can
through our nature alone be clearly and distinctly understood.
On the other hand, I say that we are passive as regards something
when that something takes place within us, or follows from our
nature externally, we being only the partial cause.

III. By emotion I mean the modifications of the body, whereby
the active power of the said body is increased or diminished,
aided or constrained, and also the ideas of such modifications.

N.B. If we can be the adequate cause of any of these
modifications, I then call the emotion an activity, otherwise I
call it a passion, or state wherein the mind is passive.

POSTULATES

I. The human body can be affected in many ways, whereby its
power of activity is increased or diminished, and also in other
ways which do not render its power of activity either greater or
less.

N.B. This postulate or axiom rests on Postulate i. and
Lemmas v. and vii., which see after II. xiii.