Causation, then, is that mode or aspect of the Uniformity of Nature which especially concerns us in Induction; and we must make it as definite as possible.  It is nothing occult, but merely a convenient name for phenomena in a particular relation to other phenomena, called their effect.  Similarly, if the word ‘force’ is sometimes used for convenience in analysing causation, it means nothing more than something in time and space, itself moving, or tending to move, or hindering or accelerating other things.  If any one does not find these words convenient for the purpose, he can use others.

Sec. 2.  A Cause, according to Mill, is “the invariable unconditional antecedent” of a given phenomenon.  To enlarge upon this: 

(1) A Cause is relative to a given phenomenon, called the Effect.  Logic has no method for investigating the cause of the universe as a whole, but only of a part or epoch of it:  we select from the infinite continuum of Nature any portion that is neither too large nor too small for a trained mind to comprehend.  The magnitude of the phenomenon may be a matter of convenience.  If the cause of disease in general be too wide a problem, can fevers be dealt with; or, if that be too much, is typhus within the reach of inquiry?  In short, how much can we deal with accurately?

(2) The given phenomenon is always an event; that is to say, not a new thing (nothing is wholly new), but a change in something, or in the relative position of things.  We may ask the cause of the phases of the moon, of the freezing of water, of the kindling of a match, of a deposit of chalk, of the differentiation of species.  To inquire the cause of France being a republic, or Russia an autocracy, implies that these countries were once otherwise governed, or had no government:  to inquire the cause of the earth being shaped like an orange, implies that the matter of the earth had once another shape.

(3) The Cause is antecedent to the Effect, which accordingly is often called its consequent.  This is often misunderstood and sometimes disputed.  It has been said that the meaning of ‘cause’ implies an ‘effect,’ so that until an effect occurs there can be no cause.  But this is a blunder; for whilst the word ‘cause’ implies ‘effect,’ it also implies the relative futurity of the effect; and effect implies the relative priority of the cause.  The connotation of the words, therefore, agrees well enough with Mill’s doctrine.  In fact, the danger is that any pair of contrasted words may suggest too strongly that the phenomena denoted are separate in Nature; whereas every natural process is continuous.  If water, dripping from the roof wears away a stone, it fell on the roof as rain; the rain came from a condensing cloud; the cloud was driven by the wind from the sea, whence it exhaled; and so on.  There is no known beginning to this, and no break in it.  We may take any one of these changes, call it an effect, and ask for its cause; or call it a cause, and ask for its effect.  There is not in Nature one set of things called causes and another called effects; but every change is both cause (or a condition) of the future and effect of the past; and whether we consider an event as the one or the other, depends upon the direction of our curiosity or interest.