According to the precedent doctrine, there are no objects which, by the mere survey, without consulting experience, we can determine to be the causes of any other; and no objects which we can certainly determine in the same manner not to be the causes. Any thing may produce any thing. Creation, annihilation, motion, reason, volition; all these may arise from one another, or from any other object we can imagine. Nor will this appear strange if we compare two principles explained above, _that the constant conjunction of objects determines their causation_,[18] and _that, properly speaking, no objects are contrary to each other but existence and non-existence_. Where objects are not contrary, nothing hinders them from having that constant conjunction on which the relation of cause and effect totally depends. Since, therefore, 'tis possible for all objects to become causes or effects to each other, it may be proper to fix some general rules by which we may know when they really are so. 1. The cause and effect must be contiguous in space and time. 2. The cause must be prior to the effect. 3. There must be a constant union betwixt the cause and effect. 'Tis chiefly this quality that constitutes the relation. 4. The same cause always produces the same effect, and the same effect never arises but from the same cause. This principle we derive from experience, and is the source of most of our philosophical reasonings. For when by any clear experiment we have discovered the causes or effects of any phenomenon, we immediately extend our observation to every phenomenon of the same kind, without waiting for that constant repetition, from which the first idea of this relation is derived. 5. There is another principle which hangs upon this, viz. that where several different objects produce the same effect, it must be by means of some quality which we discover to be common amongst them. For as like effects imply like causes, we must always ascribe the causation to the circumstance wherein we discover the resemblance. 6. The following principle is founded on the same reason. The difference in the effects of two resembling objects must proceed from that particular in which they differ. For as like causes always produce like effects, when in any instance we find our expectation to be disappointed, we must conclude that this irregularity proceeds from some difference in the causes.