es; but if it could be shown
that the elevation was the only antecedent changed in a single
instance, causal connexion was established between this and the
phenomenon of the fall of the barometer.

It is obvious that in coming to this conclusion we assume what cannot
be demonstrated but must simply be taken as a working principle to be
confirmed by its accordance with experience, that nothing comes into
being without some change in the antecedent circumstances. This is the
assumption known as the Law of Causation--_ex nihilo nihil fit_.

Again, certain observable facts are taken as evidence that there is
no causal connexion. On the assumption that any antecedent in whose
absence a phenomenon takes place is not causally connected with it,
we set aside or eliminate various antecedents as fortuitous or
non-causal. This negative principle, as we shall see, is the
foundation of what Mill called the Method of Agreement.

Be it remarked, once for all, that before coming to a conclusion on
the Positive Method or Method of Difference, we may often have to make
many observations on the Negative Method. Thus Pascal's experimenters,
before concluding that the change of altitude was the only influential
change, tried the barometer in exposed positions and in sheltered,
when the wind blew and when it was calm, in rain and in fog, in order
to prove that these circumstances were indifferent. We must expound
and illustrate the methods separately, but every method known to
science may have in practice to be employed in arriving at a single
conclusion.

[Footnote 1: This is implied, as I have already remarked, in
the word Experimental. An experiment is a proof or trial: of
what? Of a theory, a conjecture.]

[Footnote 2: If we remember, as becomes apparent on exact
psychological analysis, that things and their qualities are as
much _noumena_ and not, strictly speaking, _phenomena_ as the
attraction of gravity or the quaquaversus principle in liquid
pressure, the prejudice against occultism is mitigated.]

[Footnote 3: The modification was that causation is not only
"invariable" but also "unconditional" sequence. This addition
of unconditionality as part of the meaning of cause, after
defining cause as the sum total of the conditions, is very
much like arguing in a circle. After all, the only point
recognised in the theory as observable is the invariability of
the sequence.