An Enquiry Concerning Human Understanding

David Hume · English
[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
    loose meanings annexed to them; and their ideas are very
    uncertain and confused. No animal can put external bodies in
    motion without the sentiment of a _nisus_ or endeavour; and
    every animal has a sentiment or feeling from the stroke or blow
    of an external object, that is in motion. These sensations,
    which are merely animal, and from which we can _à priori_ draw
    no inference, we are apt to transfer to inanimate objects, and
    to suppose, that they have some such feelings, whenever they
    transfer or receive motion. With regard to energies, which are
    exerted, without our annexing to them any idea of communicated
    motion, we consider only the constant experienced conjunction
    of the events; and as we _feel_ a customary connexion between
    the ideas, we transfer that feeling to the objects; as nothing
    is more usual than to apply to external bodies every internal
    sensation, which they occasion.