III. (b) If two phenomena, having the other marks of cause and effect, seem unequal, the less contains an unexplored factor.

III. (c) If an antecedent and consequent do not increase or decrease correspondingly, they are not cause and effect, so far as they vary.

It will next be shown that these propositions are variously combined in Mill’s five Canons of Induction:  Agreement, the Joint Method, Difference, Variations, Residues.  The first three are sometimes called Qualitative Methods, and the two last Quantitative; and although this grouping is not quite accurate, seeing that Difference is often used quantitatively, yet it draws attention to an important distinction between a mere description of conditions and determination by exact measurement.

To avoid certain misunderstandings, some slight alterations have been made in the wording of the Canons.  It may seem questionable whether the Canons add anything to the above propositions:  I think they do.  They are not discussed in the ensuing chapter merely out of reverence for Mill, or regard for a nascent tradition; but because, as describing the character of observations and experiments that justify us in drawing conclusions about causation, they are guides to the analysis of observations and to the preparation of experiments.  To many eminent investigators the Canons (as such) have been unknown; but they prepared their work effectively so far only as they had definite ideas to the same purport.  A definite conception of the conditions of proof is the necessary antecedent of whatever preparations may be made for proving anything.

CHAPTER XVI

THE CANONS OF DIRECT INDUCTION

Sec. 1.  Let me begin by borrowing an example from Bain (Logic:  B. III. c. 6).  The North-East wind is generally detested in this country:  as long as it blows few people feel at their best.  Occasional well-known causes of a wind being injurious are violence, excessive heat or cold, excessive dryness or moisture, electrical condition, the being laden with dust or exhalations.  Let the hypothesis be that the last is the cause of the North-East wind’s unwholesome quality; since we know it is a ground current setting from the pole toward the equator and bent westward by the rotation of the earth; so that, reaching us over thousands of miles of land, it may well be fraught with dust, effluvia, and microbes.  Now, examining many cases of North-East wind, we find that this is the only circumstance in which all the instances agree:  for it is sometimes cold, sometimes hot; generally dry, but sometimes wet; sometimes light, sometimes violent, and of all electrical conditions.  Each of the other circumstances, then, can be omitted without the N.E. wind ceasing to be noxious; but one circumstance is never absent, namely, that it is a ground current.  That circumstance, therefore, is probably the cause of its injuriousness.  This case illustrates:—­