is we desire to know, when we endeavour at a conception of it. We say,
for instance, that the vibration of this string is the cause of this
particular sound. But what do we mean by that affirmation? We either
mean _that this vibration is followed by this sound, and that all
similar vibrations have been followed by similar sounds:_ Or, _that this
vibration is followed by this sound, and that upon the appearance of one
the mind anticipates the senses, and forms immediately an idea of the
other._ We may consider the relation of cause and effect in either of
these two lights; but beyond these, we have no idea of it.[16]

[16] According to these explications and definitions, the idea
of _power_ is relative as much as that of _cause;_ and both
have a reference to an effect, or some other event constantly
conjoined with the former. When we consider the _unknown_
circumstance of an object, by which the degree or quantity of
its effect is fixed and determined, we call that its power: And
accordingly, it is allowed by all philosophers, that the effect
is the measure of the power. But if they had any idea of power,
as it is in itself, why could not they Measure it in itself?
The dispute whether the force of a body in motion be as its
velocity, or the square of its velocity; this dispute, I say,
need not be decided by comparing its effects in equal or
unequal times; but by a direct mensuration and comparison.

As to the frequent use of the words, Force, Power, Energy, &c.,
which every where occur in common conversation, as well as in
philosophy; that is no proof, that we are acquainted, in any
instance, with the connecting principle between cause and
effect, or can account ultimately for the production of one
thing to another. These words, as commonly used, have very