To justify inductively our belief in causation, the instances compared must agree, or differ, in one circumstance only (besides the effect).  The flowing tide is an antecedent of the ebbing tide; it is invariably so, and is equal to it; but it is not the cause of it:  other circumstances are present; and the moon is the chief condition of both flow and ebb.  In several instances, States that have grown outrageously luxurious have declined in power:  that luxury caused their downfall may seem obvious, and capable of furnishing a moral lesson to the young.  Hence other important circumstances are overlooked, such as the institution of slavery, the corruption and rapacity of officials and tax-gatherers, an army too powerful for discipline; any or all of which may be present, and sufficient to explain both the luxury and the ruin.

(6) To mistake one condition of a phenomenon for the whole cause.  To speak of an indispensable condition of any phenomenon as the cause of it, may be a mere conventional abbreviation; and in this way such a mode of expression is common not only in popular but also in scientific discussion.  Thus we say that a temperature of 33 deg.  F. is a cause of the melting of ice; although that ice melts at 33 deg.  F., must further depend upon something in the nature of water; for every solid has its own melting-point.  As long, then, as we remember that ‘cause,’ used in this sense, is only a convenient abbreviation, no harm is done; but, if we forget it, fallacy may result:  as when a man says that the cause of a financial crisis was the raising of the rate of discount, neglecting the other conditions of the market; whereas, in some circumstances, a rise of the Bank-rate may increase public confidence and prevent a crisis.

We have seen that the direct use of the Canons of Agreement and Difference may only enable us to say that a certain antecedent is a cause or an indispensable condition of the phenomenon under investigation.  If, therefore, it is important to find the whole cause, we must either experiment directly upon the other conditions, or resort to the Method of Residues and deductive reasoning; nor must we be content, without showing (where such precision is possible) that the alleged cause and the given phenomenon are equal.

(7) To mistake a single consequence of a given cause for the whole effect, is a corresponding error; and none so common.  Nearly all the mistakes of private conduct and of legislation are due to it:  To cure temporary lassitude by a stimulant, and so derange the liver; to establish a new industry by protective duties, and thereby impoverish the rest of the country; to gag the press, and so drive the discontented into conspiracy; to build an alms-house, and thereby attract paupers into the parish, raise the rates, and discourage industry.