derivative action. And this observed regularity it is found possible to
trace much more widely than it is possible to trace any clear evidence
of what we understand by force. And so, at last, we frequently use the
word force as it were by anticipation, not to express the cause of the
phenomena, which indeed we do not yet know, but as a convenient
abbreviation for a large number of facts classed under one head. And
this it is which enables Hume to maintain that we mean no more by a
cause than an event which is invariably followed by another event. We
discover invariability much faster than we can discover causation; and
having discovered invariability in any given case, we presume causation
even when we cannot yet show it, and use language in accordance with
that presumption. Thus, for instance, we speak of the force of
gravitation, although we cannot yet prove that there is any such force,
and all that we know is that material particles move as if such a force
were acting on them.

As Science advances it is seen that the regularity of phenomena is far
more important to us than their causes. And the attention of all
students of Nature is fixed on that rather than on causation. And this
regularity is seen to be more and more widely pervading all phenomena of
every class, until the mind is forced to conceive the possibility that
it may be absolutely universal, and that even will itself may come
within its supreme dominion.

But to the very last the idea of causation retains the traces of its
origin. For in the first place every step in this building up of science
assumes a permanence underlying all phenomena. We cannot believe that
the future will be like the past except because we believe that there is
something permanent which was in the past and will be in the future. And
this assumption of something permanent in things around us comes from