they are commonly found upon examination to be linked by a chain of causes, which are contiguous among themselves, and to the distant objects; and when in any particular instance we cannot discover this connexion, we still presume it to exist.  We may therefore consider the relation of contiguity as essential to that of causation; at least may suppose it such, according to the general opinion, till we can find a more [Part IV.  Sect. 5.] proper occasion to clear up this matter, by examining what objects are or are not susceptible of juxtaposition and conjunction.

The second relation I shall observe as essential to causes and effects, is not so universally acknowledged, but is liable to some controversy.  It is that of priority Of time in the cause before the effect.  Some pretend that it is not absolutely necessary a cause should precede its effect; but that any object or action, in the very first moment of its existence, may exert its productive quality, and give rise to another object or action, perfectly co-temporary with itself.  But beside that experience in most instances seems to contradict this opinion, we may establish the relation of priority by a kind of inference or reasoning.  It is an established maxim both in natural and moral philosophy, that an object, which exists for any time in its full perfection without producing another, is not its sole cause; but is assisted by some other principle, which pushes it from its state of inactivity, and makes it exert that energy, of which it was secretly possest.  Now if any cause may be perfectly co-temporary with its effect, it is certain, according to this maxim, that they must all of them be so; since any one of them, which retards its operation for a single moment, exerts not itself at that very individual time, in which it might have operated; and therefore is no proper cause.  The consequence of this would be no less than the destruction of that succession of causes, which we observe in the world; and indeed, the utter annihilation of time.  For if one cause were co-temporary with its effect, and this effect with its effect, and so on, it is plain there would be no such thing as succession, and all objects must be co-existent.

If this argument appear satisfactory, it is well.  If not, I beg the reader to allow me the same liberty, which I have used in the preceding case, of supposing it such.  For he shall find, that the affair is of no great importance.

Having thus discovered or supposed the two relations of contiguity and succession to be essential to causes and effects, I find I am stopt short, and can proceed no farther in considering any single instance of cause and effect.  Motion in one body is regarded upon impulse as the cause of motion in another.  When we consider these objects with utmost attention, we find only that the one body approaches the other; and that the motion of it precedes that of the other, but without any, sensible interval.  It is in vain to rack ourselves with farther thought and reflection upon this subject.  We can go no farther in considering this particular instance.