The comprehension of the quantitative aspect of causation is greatly aided by Bain’s analysis of any cause into a ’Moving or an Inciting Power’ and a ‘Collocation’ of circumstances.  When a demagogue by making a speech stirs up a mob to a riot, the speech is the moving or inciting power; the mob already in a state of smouldering passion, and a street convenient to be wrecked, are the collocation.  When a small quantity of strychnine kills a man, the strychnine is the inciting power; the nature of his nervo-muscular system, apt to be thrown into spasms by that drug, and all the organs of his body dependent on that system, are the collocation.  Now any one who thinks only of the speech, or the drug, in these cases, may express astonishment at the disproportion of cause and effect: 

    “What great events from trivial causes spring!”

But, remembering that the whole cause of the riot included the excited mob, every one sees that its muscular power is enough to wreck a street; and remembering that breathing depends upon the normal action of the intercostal muscles, it is plain that if this action is stopped by strychnine, a man must die.  Again, a slight rise of temperature may be a sufficient inciting power to occasion extensive chemical changes in a collocation of elements otherwise stable; a spark is enough to explode a powder magazine.  Hence, when sufficient energy to account for any effect cannot be found in the inciting power, or manifestly active condition, we must look for it in the collocation which is often supposed to be passive.

And that reminds us of another common misapprehension, namely, that in Nature some things are passive and others active:  the distinction between ‘agent’ and ‘patient.’  This is a merely relative distinction:  in Nature all things are active.  To the eye some things seem at rest and others in motion; but we know that nothing is really at rest, that everything palpitates with molecular change, and whirls with the planet through space.  Everything that is acted upon reacts according to its own nature:  the quietest-looking object (say, a moss-covered stone), if we try to push or lift it, pushes or pulls us back, assuring us that ‘action and reaction are equal and opposite.’  ‘Inertia’ does not mean want of vigour, but may be metaphorically described as the inexpugnable resolve of everything to have its own way.

The equality of cause and effect defines and interprets the unconditionality of causation.  The cause, we have seen, is that group of conditions which, without any further condition, is followed by a given event.  But how is such a group to be conceived?  Unquantified, it admits only of a general description:  quantified, it must mean a group of conditions equal to the effect in mass and energy, the essence of the physical world.  Apparently, a necessary conception of the human mind:  for if a cause seem greater than its effect, we ask what has become of the surplus matter and energy; or if an effect seem greater than its cause, we ask whence the surplus matter and energy has arisen.  So convinced of this truth is every experimenter, that if his results present any deviation from it, he always assumes that it is he who has made some mistake or oversight, never that there is indeterminism or discontinuity in Nature.