With respect to coexistent emotions produced by different causes or objects, it must be observed, that there cannot be a concordance among objects of sight like what is perceived in sounds. Objects of sight are never mixed or incorporated in the act of vision. Each object is perceived as it exists, separately from others; and each raiseth its own emotion, which is felt distinctly however intimately connected the objects may be. This doctrine holds in all the causes of emotion or passion, sounds only excepted. To explain the manner in which such emotions coexist, similar emotions must be distinguished from those that are dissimilar. Two emotions are said to be similar, when they tend each of them to produce the same tone of mind. Chearful emotions, however different their causes may be, are similar; and so are those which are melancholy. Dissimilar emotions are easily explained by their opposition to what are similar. Grandeur and littleness, gaiety and gloominess, are dissimilar emotions. Emotions perfectly similar, readily combine and unite[36], so as in a manner to become one complex emotion; witness the emotions produced by a number of flowers in a parterre, or of trees in a wood. Emotions again that are opposite or extremely dissimilar, never combine nor unite. The mind cannot simultaneously take on opposite tones: it cannot at the same instant be both joyful and sad, angry and satisfied, proud and humble. Dissimilar emotions may succeed each other with rapidity, but they cannot exist simultaneously. Betwixt these two extremes, emotions will unite more or less, in proportion to the degree of their resemblance and the greater or less connection of their causes. The beauty of a landscape and the singing of birds, produce emotions that are similar in a considerable degree; and these emotions therefore, though proceeding from very different causes, readily combine and unite. On the other hand, when the causes are intimately connected, the emotions, though but slightly resembling each other, are forced into a sort of union. I give for an example a mistress in distress. When I consider her beauty, I feel a pleasant emotion; and a painful emotion when I consider her distress. These two emotions, proceeding from different views of the object, have very little resemblance to each other: and yet their causes are so intimately connected, as to force them into a sort of complex emotion, partly pleasant partly painful. This clearly explains some expressions common in poetry, _a sweet distress, a pleasant pain_.