Sometimes the separate tendencies of combined forces can only be theoretically distinguished:  as when the motion of a projectile is analysed into a tendency to travel in the straight line of its discharge, and a tendency to fall straight to the ground.  But sometimes a tendency can be isolated:  as when,—­after dropping a feather in some place sheltered from the wind, and watching it drift to and fro, as the air, offering unequal resistances to its uneven surface, counteracts its weight with varying success, until it slowly settles upon the ground,—­we take it up and drop it again in a vacuum, when it falls like lead.  Here we have the tendency of a certain cause (namely, the relation between the feather and the earth) free from counteraction:  and this is called the Elimination of the counteracting circumstances.  In this case indeed there is physical elimination; whereas, in the case of a projectile, when we say that its actual motion is resolvable (neglecting the resistance of the air) into two tendencies, one in the line of discharge, the other earthwards, there is only theoretical elimination of either tendency, considered as counteracting the other; and this is more specifically called the Resolution or Analysis of the total effect into its component conditions.  Now, Elimination and Resolution may be said to be the essential process of Induction in the widest sense of the term, as including the combination of Induction with Deduction.

The several conditions constituting any cause, then, by aiding or counteracting one another’s tendencies, jointly determine the total effect.  Hence, viewed in relation one to another, they may be said to stand in Reciprocity or mutual influence.  This relation at any moment is itself one of co-existence, though it is conceived with reference to a possible effect.  As Kant says, all substances, as perceived in space at the same time, are in reciprocal activity.  And what is true of the world of things at any moment (as connected, say, by gravity), is true of any selected group of circumstances which we regard as the particular cause of any event to come.  The use of the concept of reciprocity, then, lies in the analysis of a cause:  we must not think of reciprocity as obtaining in the succession of cause and effect, as if the effect could turn back upon its cause; for as the effect arises its cause disappears, and is irrecoverable by Nature or Magic.  There are many cases of rhythmic change and of moving equilibria, in which one movement or process produces another, and this produces something closely resembling the former, and so on in long series; as with the swing of a pendulum or the orbit of a planet:  but these are series of cause and effect, not of reciprocity.

CHAPTER XV

INDUCTIVE METHOD