Do not primary and secondary qualities in Philosophy put your Lordship in mind of two partners in a Country Dance? They stand cheek by joul for a long time very cordially. Then they set, cast off, and turn their backs as if mutually affronted and never to see one another any more. After some time they meet as good friends as ever; set, cast off again, and so on to the end of the Dance, when they are found hand to hand, in perfect friendship.
In the first ages of the world when Common Sense reigned uncontrouled by the subtleties of philosophy, primary and secondary qualities dwelt peaceably under the same roof and were joynt proprietors of the same subject, body. Democritus and Epicurus set them at variance. And pretending to find out that secondary qualities were mere spectres and illusions of sense, they banished them to Fairy Land. Aristotle took compassion upon them, recalled them, and restored them to their former Inheritance. And during his Administration they dwelt cordially with the primary, and the distinction betwixt the one and the other was forgot.
DesCartes, Malebranche, & Locke set them at variance again, but were not so cruel to the secondary qualities as their predecessors in atomical philosophy had been. They indeed turned them out of body, but seemed to make ample amends by giving them a place in the mind, and making them sensations of the mind. And now one would think that both parties must remain ever satisfied and never more have any thing to do with each other. But Fate had ordained that they should not dwell long asunder, and like true lovers they both soon repent of the separation.
For First, the secondary qualities, far from being vain of their new possession, seemed to disdain being called by its name; and retaining a strange and unaccountable liking to the old material subject, and to their old companions, still retained the name and title of Secondary Qualities of Body, even among those who believed that they had no part nor lot in body, but in the mind onely.
Secondly, the primary qualities, now in the sole and undisturbed possession of the material subject, were so far from being satisfied with their condition, that unable to brook a longer separation, they took a desparate resolution, unparalleled in history, & in a body forsook their native country and inheritance to follow their old companions.
This extraordinary event your Lordship knows happened in the aera and under the Philosophical Administration of Bishop Berkley. The good Bishop did the best he could to accommodate both primary and secondary qualities with lodgings in the human mind. And having both become meer sensations, they lost their distinction again. In the mean time, the old material subject, which had made so magnificent a figure in ages past, being now stript of all its qualities, looked so pitifull, that it is said to have sunk into the Abyss to cover the shame of its nakedness.
The next aera again disjoyned primary and secondary qualities, not onely from one another but from every subject. For having now no where to lay their head but in the human mind; a bold and cunning Engineer (by what motive induced to so daring an attempt remains a secret) dexterously sapped the foundation of this edifice, untill at last the substratum cracked and gave way, and the whole superstructure fell to pieces. Primary and secondary qualities as well [as] all the other inhabitants who survived the Ruin, were left with Epicurus' Atoms, vacuum per inane vagari.
In this situation of things your Lordship acts charitably as well as justly, by restoring both primary and secondary qualities to their ancient inheritance, to which I think they had always a just title.
Thomas Reid, The Correspondence of Thomas Reid, Paul Wood, ed. Edinburgh University Press (Edinburgh: 2002) pp. 27-28. This is in a letter to Lord Kames (14 February 1763). The bold and cunning Engineer is Hume, of course. I'm not really sure why Reid thinks Kames is reuniting the partners. Kames holds that primary qualities are in the objects and secondary qualities in the mind, so he actually divides them. But he does hold that we have an unavoidable mental impulse to treat secondary qualities as if they were also in the object, and that this illusion is, as he calls it, "the cement of society, connecting men and things together in an amiable union" (Essay on the Principles of Morality and Natural Religion, Part II, Section III), so perhaps Reid thinks this enough.
