Book 3, Chapter 10, Footnote #02
On the possibility of using Concomitant Variations
It is justly remarked by Professor Bain, that though the Methods of Agreement and Difference are not applicable to these cases, they are not wholly inaccessible to the Method of Concomitant Variations. ``If a cause happens to vary alone, the effect will also vary alone: a cause and effect may be thus singled out under the greatest complications. Thus, when the appetite for food increases with the cold, we have a strong evidence of connection between these two facts, although other circumstances may operate in the same direction. The assigning of the respective parts of the sun and moon in the action of the tide, may be effected, to a certain degree of exactness, by the variations of the amount according to the positions of the two attractive bodies. By a series of experiments of Concomitant Variations, directed to ascertain the elimination of nitrogen from the human body under varieties of muscular exercise, Dr. Parkes obtained the remarkable conclusion that a muscle grows during exercise, and loses bulk during the subsequent rest.''---Logic, ii. p. 83.
It is, no doubt, often possible to single out the influencing causes from among a great number of mere concomitants, by noting what are the antecedents a variation in which is followed by a variation in the effect. But when there are many influencing causes, no one of them greatly predominating over the rest, and especially when some of these are continually changing, It is scarcely ever possible to trace much a relation between the variations of the effect and those of any one cause as would enable us to assign to that cause its real share in the production of the effect.