Subject to the conditions thus stated, the rules may be briefly put as follows:
I. (a) That which (without further change) is followed by a given event is its cause.
II. (a) That which is not so followed is not the cause.
I. (b) That which cannot be left out without impairing a phenomenon is a condition of it.
II. (b) That which can be left out is not a condition of it.
B. Quantitative Determination
The Equality of Cause and Effect may be further explained by these rules:
III. (a) When a cause (or effect) increases or decreases, so does its effect (or cause).
III. (b) If two phenomena, having the other marks of cause and effect, seem unequal, the less contains an unexplored factor.