invariability
(since causation is uniform), though no other instances should ever be
obtainable; because it establishes once for all the unconditionality of
the connection

A B C
_p q r_.

Now, formally this is true; but in any actual investigation how shall we
decide what is a satisfactory or perfect experiment? Such an experiment
requires that in the negative instance

B C
_q r_,

BC shall be the least assemblage of conditions necessary to co-operate
with A in producing _p_; and that it is so cannot be ascertained without
either general prior knowledge of the nature of the case or special
experiments for the purpose. So that invariability will not really be
inferred from a single experiment; besides that every prudent inquirer
repeats his experiments, if only to guard against his own liability to
error.

The supposed plurality of causes does not affect the method of
Difference. In the above symbolic case, A is clearly _one_ cause (or
condition) of _p_, whatever other causes may be possible; whereas with
the Single Method of Agreement, it remained doubtful (admitting a
plurality of causes) whether A, in spite of being always present with
_p_, was ever a cause or condition of it.

This method of Difference without our being distinctly aware of it, is
oftener than any other the basis of ordinary judgments. That the sun
gives light and heat, that food nourishes and fire burns, that a stone
breaks a window or kills a bird, that the turning of a tap permits or
checks the flow of water or of gas, and thousands of other propositions
are known to be true by rough but often emphatic applications of this
method in common experience.

The method of Difference may be applied either (1) by observation, on
finding two instances (distinct assemblages of conditions) differing
only in one phenomenon together with its antecedent or consequent; or
(2) by experiment, and then, either (a) by preparing two instances that
may be compared side by side, or (b) by taking certain conditions, and
then introducing (or subtracting) some agent, supposed to be the cause,
to see what happens: in the latter case the "two instances" are the same
assemblage of conditions considered before and, again, after, the
introduction of the agent. As an example of (a) there is an experiment
to show that radium gives off heat: take two glass tubes, in one put
some chloride of radium, in both thermometers, and close them with
cotton-wo