Lastly, it will afford us another general view, to consider the difference betwixt an original propensity and an affection produced by custom. The former adheres too close to the constitution ever to be eradicated; and for that reason the passions to which it gives birth, endure during life with no remarkable diminution of strength. The latter, which owes its birth and increment to time, owes its decay to the same cause. Affection decays gradually as it grew. Hence long absence extinguisheth hatred as well as love. Affection wears out more gradually betwixt persons, who, living together, are objects to each other of mutual good-will and kindness. But here habit comes in luckily, to supply decayed affection. It makes these persons necessary to the happiness of each other, by the pain of separation[35]. Affection to children hath a long endurance, longer perhaps than any other affection. Its growth keeps pace with that of its objects. They display new beauties and qualifications daily, to feed and augment the affection. But whenever the affection becomes stationary, it must begin to decay; with a slow pace indeed, in proportion to its increment. In short, man with respect to this life, is a temporary being. He grows, becomes stationary, decays; and so must all his powers and passions. PART IV. _Coexistent emotions and passions._ To have a thorough knowledge of the human passions and emotions, it is not sufficient that they be examined singly and separately. As a plurality of them are sometimes felt at the same instant, the manner of their coexistence, and the effects thereby produced, ought also to be examined. This subject is extensive, and it will be difficult to evolve all the laws that govern its endless variety of cases. Such an undertaking may be brought to perfection, but it must be by degrees. The following hints may suffice for a first attempt. We begin with emotions raised by different sounds, as the simplest case. Two sounds that mix, and are, as it were, incorporated before they reach the ear, are said to be concordant. That each sound produceth an emotion of its own, must be admitted. But then these emotions, like the sounds that produce them, mix so intimately, as to be rather one complex emotion than two emotions in conjunction. Two sounds, again, that refuse incorporation or mixture, are said to be discordant. Being however heard at the same instant, the emotions produced by them are conjoined; and in that condition are unpleasant, even where separately they are each of them pleasant. Similar to the emotion raised by mixed sounds, is the emotion that an object of sight raises by means of its several qualities. A tree, for example, with its qualities of colour, figure, size, _&c._ is perceived to be one object; and the emotion it raises is one, not different emotions combined. But though the emotion be one, it is however not simple. The perception of the tree is complex, and the emotion raised by it must also be complex.