We proceed to the effects produced by means of the different manners of coexistence above described; first, the effects produced within the mind, and next, those that appear externally. I discover two mental effects clearly distinguishable from each other. The one may be represented by addition and subtraction in numbers, and the other by harmony in sounds. Two pleasant emotions that are similar, readily unite when they are coexistent; and the pleasure felt in the union, is the sum of the two pleasures. The combined emotions are like multiplied effects from the co-operation of different powers. The same emotions in succession, are far from making the same figure; because the mind at no instant of the succession is conscious of more than a single emotion. This doctrine may aptly be illustrated by a landscape comprehending hills, vallies, plains, rivers, trees, _&c._ The emotions produced by these several objects, being similar in a high degree as falling in easily and sweetly with the same tone of mind, are in conjunction extremely pleasant. And this multiplied effect is felt from objects even of different senses; as where a landscape is conjoined with the music of birds and odor of flowers. Such multiplied effect, as above hinted, depends partly on the resemblance of the emotions and partly on the connection of their causes; whence it follows, that the effect must be the greatest, where the causes are intimately connected and the emotions perfectly similar. The other pleasure arising from coexistent emotions, which may be termed _the pleasure of concord or harmony_, is ascertained by a different rule. It is directly in proportion to the degree of resemblance betwixt the emotions, and inversely in proportion to the degree of connection betwixt the causes. To feel this pleasure in perfection, the resemblance cannot be too strong, nor the connection too slight. Where the causes are intimately connected, the similar emotions they produce are felt like one complex emotion. But the pleasure of harmony, is not felt from one emotion single or complex. It is felt from various similar emotions, distinct from each other, and yet sweetly combining in the mind; and the less connection the causes have, the more entire is the emotion of harmony. This matter cannot be better illustrated, than by the foregoing example of a landscape, where the sight, hearing, and smelling, are employed. The accumulated pleasure of so many different similar emotions, is not what delights us the most in this combination of objects. The sense of harmony from these emotions sweetly uniting in the mind, is still more delightful. We feel this harmony in the different emotions proceeding from the visible objects; but we feel it still more sensibly in the emotions proceeding from the objects of different senses. This emotion of concord or harmony, will be more fully illustrated, when the emotions produced by the sound of words and their meaning are taken under consideration[37].