But this is less important than the fact that
in his canons of the Experimental Methods Mill recognised that
more is observable.]

CHAPTER IV.

METHODS OF OBSERVATION.--SINGLE DIFFERENCE.

I.--THE PRINCIPLE OF SINGLE DIFFERENCE.--MILL'S "CANON".

On what principle do we decide, in watching a succession of phenomena,
that they are connected as cause and effect, that one happened
in consequence of the happening of another? It may be worded as
follows:--

_When the addition of an agent is followed by the appearance
or its subtraction by the disappearance of a certain effect,
no other influential circumstance having been added or
subtracted at the same time or in the meantime, and no change
having occurred among the original circumstances, that agent
is a cause of the effect._

On this principle we would justify our belief in the causal properties
of common things--that fire burns, that food appeases hunger, that
water quenches thirst, that a spark ignites gunpowder, that taking
off a tight shoe relieves a pinched foot. We have observed the
effect following when there was no other change in the antecedent
circumstances, when the circumstance to which we refer it was simply
added to or subtracted from the prior situation.

Suppose we doubt whether a given agent is or is not capable of
producing a certain effect in certain circumstances, how do we put it
to the proof? We add it singly or subtract it singly, taking care that
everything else remains as before, and watch the result. If we wish
to know whether a spoonful of sugar can sweeten a cup of tea, we taste
the tea without the sugar, then add the sugar, and taste again. The
isolated introduction of the agent is the proof, the experiment. If we
wish to know whether a pain in the foot is due to a tight lacing,
we relax the lacing and make no other change: if the pain then
disappears, we refer it to the lacing as the cause. The proof is the
disappearance of the pain on the subtraction of the single antecedent.

The principle on which we decide that there is causal connexion is
the same whether we make the experimental changes ourselves or merely
watch them as they occur--the only course open to us with the great
forces of nature which are beyond the power of human manipulation. In
any case we have proof of causation when we can make sure that there
was only one difference in the antecedent circumstances corresponding
to the