subject. It is
observable, that where the objects of contrary passions are presented
at once, beside the encrease of the predominant passion (which has
been already explained, and commonly arises at their first shock
or rencounter) it sometimes happens, that both the passions exist
successively, and by short intervals; sometimes, that they destroy each
other, and neither of them takes place; and sometimes that both of them
remain united in the mind. It may, therefore, be asked, by what theory
we can explain these variations, and to what general principle we can
reduce them.

When the contrary passions arise from objects entirely different, they
take place alternately, the want of relation in the ideas separating the
impressions from each other, and preventing their opposition. Thus when
a man is afflicted for the loss of a law-suit, and joyful for the birth
of a son, the mind running from the agreeable to the calamitous object,
with whatever celerity it may perform this motion, can scarcely temper
the one affection with the other, and remain betwixt them in a state of
indifference.

It more easily attains that calm situation, when the same event is of a
mixt nature, and contains something adverse and something prosperous
in its different circumstances. For in that case, both the passions,
mingling with each other by means of the relation, become mutually
destructive, and leave the mind in perfect tranquility.

But suppose, in the third place, that the object is not a compound of
good or evil, but is considered as probable or improbable in any degree;
in that case I assert, that the contrary passions will both of them be
present at once in the soul, and instead of destroying and tempering
each other, will subsist together, and produce a third impression
or affection by their union. Contrary passions are not capable of
destroying each other, except when their contrary movements exactly
rencounter, and are opposite in their direction, as well as in the
sensation they produce. This exact rencounter depends upon the relations
of those ideas, from which they are derived, and is more or less
perfect, according to the degrees of the relation. In the case of
probability the contrary chances are so far related, that they determine
concerning the existence or non-existence of the same object. But this
relation is far from being perfect; since some of the chances lie on
the side of existence, and others on that of non-existence;