Upon this head there may be started a very curious question concerning that contrariety of passions, which is our present 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; which are objects altogether incompatible.  It is impossible by one steady view to survey the opposite chances, and the events dependent on them; but it is necessary, that the imagination should run alternately from the one to the other.  Each view of the imagination produces its peculiar passion, which decays away by degrees, and is followed by a sensible vibration after the stroke.  The incompatibility of the views keeps the passions from shocking in a direct line, if that expression may be allowed; and yet their relation is sufficient to mingle their fainter emotions.  It is after this manner that hope and fear arise from the different mixture of these opposite passions of grief and joy, and from their imperfect union and conjunction.