The train of thought depends not entirely upon relations: another cause comes in for a share; and that is the sense of order and arrangement. To things of equal rank, where there is no room for a preference, order cannot be applied; and it must be indifferent in what manner they be surveyed; witness the sheep that make a flock, or the trees in a wood. But in things of unequal rank, order is a governing principle. Thus our tendency is, to view the principal subject before we descend to its accessories or ornaments, and the superior before the inferior or dependent. We are equally averse to enter into a minute consideration of constituent parts, till the thing be first surveyed as a whole. In passing from a part to the whole, and from an accessory to its principal, the connection is the same as in the opposite direction. But a sense of order aids the transition in the latter case, and a sense of disorder obstructs it in the former. It needs scarce be added, that in thinking or reflecting on any of these particulars, and in passing from one to another ideally, we are sensible of easiness or difficulty precisely as when they are set before our eyes. Our sense of order is conspicuous with respect to natural operations; for it always coincides with the order of nature. Thinking upon a body in motion, we follow its natural course. The mind falls with a heavy body, descends with a river, and ascends with flame and smoke. In tracing out a family, we incline to begin at the founder, and to descend gradually to his latest posterity. On the contrary, musing on a lofty oak, we begin at the trunk, and mount from it to the branches. As to historical facts, we love to proceed in the order of time; or, which comes to the same, to proceed along the chain of causes and effects.