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Book II

Chapter XXI

Of Power.

This Idea how got.       §1. The Mind, being every day informed, by the Senses, of the
alteration of those simple Ideas, it observes in things without; and
taking notice how one comes to an end, and ceases to be, and
another begins to exist, which was not before; reflecting also on
what passes within it self, and observing a constant change of its
Ideas, sometimes by the impression of outward Objects on the
Senses, and sometimes by the Determination of its own choice; and
concluding from what it has so constantly observed to have been,
that the like Changes will for the future be made, in the same things,
by like Agents, and by the like ways, considers in one thing the
possibility, of having any of its simple Ideas changed, and in another
the possibility of making that change; and so comes by that Idea
which we call Power. Thus we say, Fire has a power to melt Gold, i.e.
to destroy the consistency of its insensible parts, and consequently
its hardness, and make it fluid; and Gold has a power to be melted;
That the Sun has a power to blanch Wax, and Wax a power to be
blanched by the Sun, whereby the Yellowness is destroy’d, and
Whiteness made to exist in its room. In which, and the like Cases,
the Power we consider is in reference to the change of perceivable
Ideas. For we cannot observe any alteration to be made in, or
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operation upon any thing, but by the observable change of its
sensible Ideas; nor conceive any alteration to be made, but by
conceiving a Change of some of its Ideas.
Locke Hum II, 21, §1, pp. 233-234