We have no
idea of this connexion, nor even any distinct notion what it 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.*
* 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.
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