diligent and prudent enquiry. As moral philosophy seems hitherto to have
received less improvement than either geometry or physics, we may
conclude, that, if there be any difference in this respect among these
sciences, the difficulties, which obstruct the progress of the former,
require superior care and capacity to be surmounted.

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
endeavoured[10] 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