With relation to involuntary causes disturbing that degree of variety which nature requires, a slight pain affecting one part of the body without variation, becomes, by its constancy and long duration, almost insupportable. The patient, sensible that the pain is not increased in degree, complains of its constancy more than of its severity, that it ingrosses his whole thoughts, and gives admission to no other object. Pain, of all feelings, seizes the attention with the greatest force; and the mind, after fruitless efforts to turn its view to objects more agreeable, must abandon itself to its tormentor. A shifting pain gives less uneasiness, because change of place contributes to variety. An intermitting pain, suffering other objects to intervene, is not increased by reiteration. Again, any single colour or sound often returning, becomes disagreeable; as may be observed in viewing a train of similar apartments painted with the same colour, and in hearing the prolonged tollings of a bell. Colour and sound varied within certain limits, though without any order, are agreeable; witness a field variegated with many colours of plants and flowers, and the various notes of birds in a thicket. Increase the number or variety, and the feeling becomes unpleasant. Thus a great variety of colours, crowded upon a small canvas or in quick succession, create an uneasy feeling, which is prevented by putting the colours at a greater distance either of place or time. A number of voices in a crowded assembly, a number of animals collected in a market, produce an unpleasant emotion; though a few of them together, or all of them in a moderate succession, would be agreeable. And because of the same excess in variety, a number of pains felt in different parts of the body, at the same instant or in a rapid succession, make an exquisite torture.