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."-- (I.p.

229.) Next, the foundation of the _method of difference_ is stated:-- "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."-- (I.p.
230.) In the succeeding paragraph the _method of concomitant variations_ is foreshadowed.
"When any object increases or diminishes with the increase or diminution of the cause, 'tis 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.