This hypothesis is founded on sufficient experience.  A man, who from any motives has entertained a resolution of performing an action, naturally runs into every other view or motive, which may fortify that resolution, and give it authority and influence on the mind.  To confirm us in any design, we search for motives drawn from interest, from honour, from duty.  What wonder, then, that pity and benevolence, malice, and anger, being the same desires arising from different principles, should so totally mix together as to be undistinguishable?  As to the connexion betwixt benevolence and love, anger and hatred, being original and primary, it admits of no difficulty.

We may add to this another experiment, viz, that benevolence and anger, and consequently love and hatred, arise when our happiness or misery have any dependance on the happiness or misery of another person, without any farther relation.  I doubt not but this experiment will appear so singular as to excuse us for stopping a moment to consider it.

Suppose, that two persons of the same trade should seek employment in a town, that is not able to maintain both, it is plain the success of one is perfectly incompatible with that of the other, and that whatever is for the interest of either is contrary to that of his rival, and so vice versa.  Suppose again, that two merchants, though living in different parts of the world, should enter into co-partnership together, the advantage or loss of one becomes immediately the advantage or loss of his partner, and the same fortune necessarily attends both.  Now it is evident, that in the first case, hatred always follows upon the contrariety of interests; as in the second, love arises from their union.  Let us consider to what principle we can ascribe these passions.

It is plain they arise not from the double relations of impressions and ideas, if we regard only the present sensation.  For takeing the first case of rivalship; though the pleasure and advantage of an antagonist necessarily causes my pain and loss, yet to counter-ballance this, his pain and loss causes my pleasure and advantage; and supposing him to be unsuccessful, I may by this means receive from him a superior degree of satisfaction.  In the same manner the success of a partner rejoices me, but then his misfortunes afflict me in an equal proportion; and it is easy to imagine, that the latter sentiment may in many cases preponderate.  But whether the fortune of a rival or partner be good or bad, I always hate the former and love the latter.

This love of a partner cannot proceed from the relation or connexion betwixt us; in the same manner as I love a brother or countryman.  A rival has almost as close a relation to me as a partner.  For as the pleasure of the latter causes my pleasure, and his pain my pain; so the pleasure of the former causes my pain, and his pain my pleasure.  The connexion, then, of cause and effect is the same in both cases; and if in the one case, the cause and effect have a farther relation of resemblance, they have that of contrariety in the other; which, being also a species of resemblance, leaves the matter pretty equal.