The necessary succession of perceptions, is a subject formerly handled, so far as it depends on the relations of objects and their mutual connections[91]. But that subject is not exhausted; and I take the liberty to introduce it a second time, in order to explain in what manner we are affected by uniformity and variety. The world we inhabit is replete with things not less remarkable for their variety than their number. These, unfolded by the wonderful mechanism of external sense, furnish the mind with many perceptions, which, joined with ideas of memory, of imagination, and of reflection, form a complete train that has not a gap or interval. This tide of objects, in a continual flux, is in a good measure independent of will. The mind, as has been observed[92], is so constituted, “That it can by no effort break off the succession of its ideas, nor keep its attention long fixt upon the same object.” We can arrest a perception in its course; we can shorten its natural duration, to make room for another; we can vary the succession by change of place or amusement; and we can in some measure prevent variety, by frequently recalling the same object after short intervals: but still there must be a succession, and a change from one thing to another. By artificial means, the succession may be retarded or accelerated, may be rendered more various or more uniform, but in one shape or other is unavoidable. The rate of succession, even when left to its ordinary course, is not always the same. There are natural causes that accelerate or retard it considerably. The first I shall mention depends on a peculiar constitution of mind. One man is distinguished from another, by no circumstance more remarkably than the movement of his train of perceptions. A cold languid temper is accompanied with a slow course of perceptions, which occasions dulness of apprehension and sluggishness in action. To a warm temper, on the contrary, belongs a quick course of perceptions, which occasions quickness of apprehension and activity in business. The Asiatic nations, the Chinese especially, are observed to be more cool and deliberate than the Europeans: may not the reason be, that heat enervates by exhausting the spirits? A certain degree of cold, such as is felt in the middle regions of Europe, by bracing the fibres, rouses the mind, and produces a brisk circulation of thought, accompanied with vigour in action. In youth there is observable a quicker succession of perceptions, than in old age. Hence in youth a remarkable avidity for variety of amusements, which in riper years give place to more uniform and more sedate occupation. This qualifies men of middle age for business, where activity is required, but with a greater proportion of uniformity than variety. In old age, a slow and languid succession makes variety unnecessary; and for that reason, the aged, in all their motions, are generally governed by an habitual uniformity.