ER, FORCE,
ENERGY, NECESSITY, CONNEXION, and PRODUCTIVE QUALITY, are all nearly
synonymous; and therefore it is an absurdity to employ any of them in
defining the rest. By this observation we reject at once all the vulgar
definitions, which philosophers have given of power and efficacy; and
instead of searching for the idea in these definitions, must look for
it in the impressions, from which it is originally derived. If it be a
compound idea, it must arise from compound impressions. If simple, from
simple impressions.

I believe the most general and most popular explication of this
matter, is to say [See Mr. Locke, chapter of power.], that finding from
experience, that there are several new productions in matter, such
as the motions and variations of body, and concluding that there must
somewhere be a power capable of producing them, we arrive at last by
this reasoning at the idea of power and efficacy. But to be convinced
that this explication is more popular than philosophical, we need but
reflect on two very obvious principles. First, That reason alone can
never give rise to any original idea, and secondly, that reason, as
distinguished from experience, can never make us conclude, that a cause
or productive quality is absolutely requisite to every beginning of
existence. Both these considerations have been sufficiently explained:
and therefore shall not at present be any farther insisted on.

I shall only infer from them, that since reason can never give rise to
the idea of efficacy, that idea must be derived from experience, and
from some particular instances of this efficacy, which make their
passage into the mind by the common channels of sensation or reflection.
Ideas always represent their objects or impressions; and vice versa,
there are some objects necessary to give rise to every idea. If we
pretend, therefore, to have any just idea of this efficacy, we must
produce some instance, wherein the efficacy is plainly discoverable to
the mind, and its operations obvious to our consciousness or sensation.
By the refusal of this, we acknowledge, that the idea is impossible and
imaginary, since the principle of innate ideas, which alone can save
us from this dilemma, has been already refuted, and is now almost
universally rejected in the learned world. Our present business,
then, must be to find some natural production, where the operation and
efficacy of a cause can be clearly conceived and comprehended by the
min