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.
(7) When any object encreases or diminishes with the encrease or diminution of its cause, it is to be regarded as a compounded effect, derived from the union of the several different effects, which arise from the several different parts of the cause.

The absence or presence of one part of the cause is here supposed to be always attended with the absence or presence of a proportionable part of the effect.

This constant conjunction sufficiently proves, that the one part is the cause of the other.