experiment, and we have to depend on observation. The phenomena may
then be so mixed together that A and _p_ seem to be merely concomitant;
so that, though connection of some sort may be rendered highly probable,
we may not be able to say which is cause and which is effect. We must
then try (as Bain says) to trace the expenditure of energy: if _p_ gains
when A loses, the course of events if from A to _p_.

Moreover, where succession cannot be traced, the method of Agreement may
point to a connection between two or more facts (perhaps as co-effects
of a remote cause) where direct causation seems to be out of the
question: e.g., that Negroes, though of different tribes, different
localities, customs, etc., are prognathous, woolly-haired and
dolichocephalic.

The Method of Agreement, then, cannot by itself prove causation. Its
chief use (as Mill says) is to suggest hypotheses as to the cause; which
must then be used (if possible) experimentally to try if it produces the
given effect. A bacillus, for example, being always found with a certain
disease, is probably the chief condition of it: give it to a guinea-pig,
and observe whether the disease appears in that animal.

Men often use arguments which, if they knew it, might be shown to
conform more or less to this canon; for they collect many instances to
show that two events are connected; but usually neglect to bring out the
negative side of the proof; so that their arguments only amount to
simple enumeration. Thus Ascham in his _Toxophilus_, insisting on the
national importance of archery, argues that victory has always depended
on superiority in shooting; and, to prove it, he shows how the Parthians
checked the Romans, Sesostris conquered a great part of the known world,
Tiberius overcame Arminius, the Turks established their empire, and the
English defeated the French (with many like examples)--all by superior
archery. But having cited these cases to his purpose, he is content;
whereas he might have greatly strengthened his proof by showing how one
or the other instance excludes other possible causes of success. Thus:
the cause was not discipline, for the Romans were better disciplined
than the Parthians; nor yet the boasted superiority of a northern
habitat, for Sesostris issued from the south; nor better manhood, for
here the Germans probably had the advantage of the Romans; nor superior
civilisation, for the Turks were less civilised than most of those they
conque