ctual energies of a people; and an ever greater number of
men, renouncing the bustle, the pleasures, and the ambitions of an
active life, devote themselves exclusively to the pursuit of abstract
truth; they set themselves to discover the causes of things, to trace
the regularity and order that may be supposed to underlie the seemingly
irregular, confused, and arbitrary sequence of phenomena. Unquestionably
the progress of civilisation owes much to the sustained efforts of such
men, and if of late years and within our own memory the pace of progress
has sensibly quickened, we shall perhaps not err in supposing that some
part at least of the acceleration may be accounted for by an increase in
the number of lifelong students.

[Sidenote: The idea of cause is simply that of invariable sequence
suggested by the observation of many particular cases of sequence.]

Now when we analyse the conception of a cause to the bottom, we find as
the last residuum in our crucible nothing but what Hume found there long
ago, and that is simply the idea of invariable sequence. Whenever we say
that something is the cause of something else, all that we really mean
is that the latter is invariably preceded by the former, so that
whenever we find the second, which we call the effect, we may infer that
the first, which we call the cause, has gone before it. All such
inferences from effects to causes are based on experience; having
observed a certain sequence of events a certain number of times, we
conclude that the events are so conjoined that the latter cannot occur
without the previous occurrence of the former. A single case of two
events following each other could not of itself suggest that the one
event is the cause of the other, since there is no necessary link
between them in the mind; the sequence has to be repeated more or less
frequently before we infer a causal connexion between the two; and this
inference rests simply on that association of ideas which is established
in our mind by the reiterated observation of the things. Once the ideas
are by dint of repetition firmly welded together, the one by sheer force
of habit calls up the other, and we say that the two things which are
represented by those ideas stand to each other in the relation of cause
and effect. The notion of causality is in short only one particular case
of the association of ideas. Thus all reasoning as to causes implies
previous observation: we reason from the observe