An Enquiry Concerning Human Understanding
49. There are no ideas, which occur in metaphysics, more obscure and uncertain, than
those of power, force, energy or necessary connexion, of which it is every moment
necessary for us to treat in all our disquisitions. We shall, therefore, endeavour, in this
section, to fix, if possible, the precise meaning of these terms, and thereby remove some
part of that obscurity, which is so much complained of in this species of philosophy.
It seems a proposition, which will not admit of much dispute, that all our ideas are
nothing but copies of our impressions, or, in other words, that it is impossible for us to
think of any thing, which we have not antecedently felt, either by our external or internal
senses. I have endeavoured10 to explain and prove this proposition, and have expressed
my hopes, that, by a proper application of it, men may reach a greater clearness and
precision in philosophical reasonings, than what they have hitherto been able to attain.
Complex ideas may, perhaps, be well known by definition, which is nothing but an
enumeration of those parts or simple ideas, that compose them. But when we have pushed
up definitions to the most simple ideas, and find still some ambiguity and obscurity; what
resource are we then possessed of? By what invention can we throw light upon these
ideas, and render them altogether precise and determinate to our intellectual view?
Produce the impressions or original sentiments, from which the ideas are copied. These
impressions are all strong and sensible. They admit not of ambiguity. They are not only
placed in a full light themselves, but may throw light on their correspondent ideas, which
lie in obscurity. And by this means, we may, perhaps, attain a new microscope or species
of optics, by which, in the moral sciences, the most minute, and most simple ideas may
be so enlarged as to fall readily under our apprehension, and be equally known with the
grossest and most sensible ideas, that can be the object of our enquiry.
50. To be fully acquainted, therefore, with the idea of power or necessary connexion, let
us examine its impression; and in order to find the impression with greater certainty, let
us search for it in all the sources, from which it may possibly be derived.
When we look about us towards external objects, and consider the operation of causes,
we are never able, in a single instance, to discover any power or necessary connexion;
any quality, which binds the effect to the cause, and renders the one an infallible
consequence of the other. We only find, that the one does actually, in fact, follow the
other. The impulse of one billiard-ball is attended with motion in the second. This is the
whole that appears to the outward senses. The mind feels no sentiment or inward
impression from this succession of objects: Consequently, there is not, in any single,
particular instance of cause and effect, any thing which can suggest the idea of power or
From the first appearance of an object, we never can conjecture what effect will result
from it. But were the power or energy of any cause discoverable by the mind, we could
foresee the effect, even without experience; and might, at first, pronounce with certainty
concerning it, by mere dint of thought and reasoning.