John Locke, answered from the texts and cited to the page.
The mind arrives at the idea of power by observing change — watching one simple idea cease and another begin, whether the occasion is an outward impression on the senses or the mind's own determination of its choice — and concluding from what it has constantly observed that like changes will be produced in the future by like agents.1 From this general idea of power, a distinction immediately follows.
Power considered as the ability to make a change I call active power; power considered as the ability to receive a change I call passive power.2 The distinction matters more than it might first appear, because common language runs the two together under the same grammatical form. When I say "I see the Moon," the verb is grammatically active, yet in that act I am not operating on the Moon at all — I am receiving the ideas of light and roundness, which I cannot avoid so long as my eyes are open and directed that way.
That reception is passive capacity, not action.3 Action proper, by contrast, is what occurs when I turn my eyes away of my own choice: there I put myself into motion by a power within myself, and that is the product of active power.4 The clearest instance of active power — the power to begin a change, not merely to transmit one already received — comes not from body but from reflection on the operations of our own minds.5
A billiard ball struck by a cue does not act; it suffers the stroke and communicates forward only the motion it received, losing in itself as much as the other ball gains.6 This gives us a very obscure idea of active power from bodies alone. Thinking, and the voluntary direction of thought, is where the clearest idea of genuinely active power resides: to bring ideas out of sight into view at one's own choice, and to compare them as one thinks fit, is active power in the proper sense.7
Powers, moreover, make a great part of our complex ideas of substances precisely because our senses cannot reach the bulk, texture, and figure of the minute parts of bodies on which their real constitutions depend; we are left to use their secondary qualities — colour, taste, smell — as the characteristic marks by which we distinguish one substance from another.8
And all those secondary qualities are themselves nothing but powers: the colour and taste of opium, no less than its soporific virtue, are mere powers depending on its primary qualities, fitted to produce different operations on different parts of our bodies.9 Active and passive powers alike, as far as we have any notice of them, terminate in sensible simple ideas — which is precisely what we should expect, given that the idea of power itself is derived from observing the alteration of perceivable ideas.10
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 so comes by that Idea which we call Power.An Essay Concerning Human Understanding, pp. 256–257
Power thus considered is twofold, viz. as able to make, or able to receive any change: The one may be called Active, and the other Passive Power.An Essay Concerning Human Understanding, pp. 256–257
this Proposition, I see the Moon, or a Star, or I feel the heat of the Sun, though expressed by a Verb Active, does not signify any Action in me whereby I operate on those Substances; but the reception of the Ideas of light, roundness, and heat, wherein I am not active but barely passive, and cannot in that position of my Eyes, or Body, avoid receiving them.An Essay Concerning Human Understanding, pp. 308–309
But when I turn my Eyes another way, or remove my Body out of the Sun-beams, I am properly active; because of my own choice, by a power within my self, I put my self into that Motion. Such an Action is the product of Active Power.An Essay Concerning Human Understanding, pp. 308–309
Bodies, by our Senses, do not afford us so clear and distinct an Idea of active Power, as we have from reflection on the Operations of our Minds.An Essay Concerning Human Understanding, pp. 257–258
when the Ball obeys the stroke of a Billiard-stick, it is not any action of the Ball, but bare passion: Also when by impulse it sets another Ball in motion, that lay in its way, it only communicates the motion it had received from another, and loses in it self so much, as the other received.An Essay Concerning Human Understanding, pp. 257–258
But to be able to bring into view Ideas out of sight, at one's own choice, and to compare which of them one thinks fit, this is an Active Power.An Essay Concerning Human Understanding, pp. 308–309
our Senses failing us, in the discovery of the Bulk, Texture, and Figure of the minute parts of Bodies, on which their real Constitutions and Differences depend, we are fain to make use of their secondary Qualities, as the characteristical Notes and Marks, whereby to frame Ideas of them in our Minds, and distinguish them one from another.An Essay Concerning Human Understanding, p. 323
For the Colour and Taste of Opium, are, as well as its soporifick or anodyne Virtues, meer Powers depending on its primary Qualities, whereby it is fitted to produce different Operations, on different parts of our Bodies.An Essay Concerning Human Understanding, p. 323
All which Powers, as far as we have any Notice or Notion of them, terminate only in sensible simple Ideas.An Essay Concerning Human Understanding, p. 323