BOOK I PART III
ern philosophers, than this concerning the ef-
ficacy of causes, or that quality which makes
them be followed by their effects. But before
they entered upon these disputes, methinks it
would not have been improper to have exam-
ined what idea we have of that efficacy, which
is the subject of the controversy. This is what
I find principally wanting in their reasonings,
and what I shall here endeavour to supply.
I begin with observing that the terms ofef-
ficacy, agency, power, force, energy, necessity, con-
nexion and productive quality, are all nearly syn-
onymous; and therefore it is an absurdity to
employ any of them in defining the rest. By
this observation we reject at once all the vul-
gar definitions, which philosophers have given
of power and efficacy; and instead of searching
for the idea in these definitions, must look for