John Locke, answered from the texts and cited to the page.
The distinction, at its root, is between what belongs to body as body, and what belongs only to the relation between body and perceiver. Primary qualities are those that a body retains through every division and alteration — solidity, extension, figure, motion or rest, and number. Divide a grain of wheat in two, and each part still has solidity, extension, figure, and mobility; divide it again, and still the same; carry the division on until the parts are too small for any sense to detect, and each insensible particle must retain all those qualities still, for division can do nothing but make two or more distinct masses of what was one before.12
These qualities are in the things themselves whether we perceive them or no, and when the bodies that have them are large enough to be discerned, the idea we receive of them is a genuine resemblance — a circle or square is the same whether in idea or in existence, in the mind or in the manna. Secondary qualities, by contrast, are nothing in the objects themselves but powers — powers, grounded in the bulk, figure, texture, and motion of insensible parts, to produce in us the various sensations of colour, sound, taste, smell, warmth, and the like.3
The sweetness you taste in manna is not in the manna as sweetness is in your idea of it; it is in the manna only as a power, operating on the palate by the size, motion, and figure of its particles, to produce that sensation in you.45 Take away the sensation — let not the eyes see light or colours, nor the ears hear sounds, nor the palate taste — and all colours, tastes, odours, and sounds, as particular ideas, vanish and cease.6
The asymmetry is the point I press hardest. The same fire that at one distance produces the sensation of warmth produces, at a nearer approach, the far different sensation of pain; and no one supposes the pain is in the fire.7 Why then should warmth be thought to be there? Both are effects of the fire's primary qualities operating on the body; the only difference is that custom has led men to project the one onto the object and not the other.
Whiteness and coldness in snow, sweetness and whiteness in manna — these are no more really in those bodies than sickness or pain is in manna when manna disorders the stomach.8 The sickness is confessedly only an effect of manna's operation on the guts by the size, motion, and figure of its insensible parts; the sweetness is an effect of the same manna operating on the palate by ways equally unknown — and there is no reason to grant one a reality in the object that is denied the other.9
These I call original or primary Qualities of Body, which I think we may observe to produce simple Ideas in us, viz. Solidity, Extension, Figure, Motion, or Rest, and Number.An Essay Concerning Human Understanding, pp. 158–159
Take a grain of Wheat, divide it into two parts, each part has still Solidity, Extension, Figure, and Mobility; divide it again, and it retains still the same qualities; and so divide it on, till the parts become insensible, they must retain still each of them all those qualities.An Essay Concerning Human Understanding, pp. 158–159
A Circle or Square are the same, whether in Idea or Existence; in the Mind, or in the Manna: And this, both Motion and Figure are really in the Manna, whether we take notice of them or no.An Essay Concerning Human Understanding, pp. 161–162
The Power that is in any Body, by Reason of its insensible primary Qualities, to operate after a peculiar manner on any of our Senses, and thereby produce in us the different Ideas of several Colours, Sounds, Smells, Tasts, etc. These are usually called sensible Qualities.An Essay Concerning Human Understanding, pp. 163–164
the Sweetness and Whiteness, effects of the same Manna on other parts of the Body, by ways equally as unknown, should be thought to exist in the Manna, when they are not seen nor tasted, would need some Reason to explain.An Essay Concerning Human Understanding, pp. 161–162
Take away the Sensation of them; let not the Eyes see Light, or Colours, nor the Ears hear Sounds; let the Palate not Taste, nor the Nose Smell, and all Colours, Tastes, Odors, and Sounds, as they are such particular Ideas, vanish and cease.An Essay Concerning Human Understanding, pp. 161–162
the same Fire, that at one distance produces in us the Sensation of Warmth, does at a nearer approach, produce in us the far different Sensation of Pain, ought to bethink himself, what Reason he has to say, That his Idea of Warmth, which was produced in him by the Fire, is actually in the Fire.An Essay Concerning Human Understanding, pp. 160–161
But Light, Heat, Whiteness, or Coldness, are no more really in them, than Sickness or Pain is in Manna.An Essay Concerning Human Understanding, pp. 160–161
the Pain and Sickness caused by Manna, are confessedly nothing, but the effects of its operations on the Stomach and Guts, by the size, motion, and figure of its insensible parts... As if it could not operate on the Eyes and Palate, and thereby produce in the Mind particular distinct Ideas, which in it self it has not.An Essay Concerning Human Understanding, pp. 161–162