Because of his connection with Edgeworth, Day joined the Lunar Society in Lichfield, despite describing himself as a philosopher rather than a scientist, and formed friendships with Erasmus Darwin and Anna Seward.

According to Anna Seward (who nevertheless observed that "there was no finding such a creature ready made"), his demands were modest: "He resolved, if possible, that his wife should have a taste for literature and science, for moral and patriotic philosophy.

He resolved also, that she should be simple as a mountain girl, in her dress, her diet and her manners, fearless and intrepid as the Spartan wives and Roman heroines.

His motives are unclear, as Day was a committed Francophobe, but leaving the country put him out of the reach of British law and the lawyers of the Foundling Hospital, who had been known to sue apprentice-masters who mistreated their apprentices.

[1] In the spring of 1770, Day decided that Lucretia, then aged 12, could not satisfy him intellectually as a wife, and returned to England, placing her in the home of a milliner's family in Ludgate Hill with a payment of £400.

According to Rousseau, who had described how children should be taught not to fear pain or physical hardship, Day embarked on a programme of 'lessons,' including dripping hot candle-wax onto Sabrina's arms and forcing her to wade into cold water up to her neck.

The contradiction between the claim that "all men are created equal" and the existence of American slavery attracted comment from some quarters when the United States Declaration of Independence was first published; Congress, having made a few changes in wording, deleted nearly a fourth of the draft before publication, most notably removing a passage critical of the slave trade, as there were members of Congress who owned black slaves.

[4] Day was among those who noted the discrepancy, writing in 1776: If there be an object truly ridiculous in nature, it is an American patriot, signing resolutions of independency with the one hand, and with the other brandishing a whip over his affrighted slaves.

Embracing Rousseau's dictates in many ways, it narrates the story of the rich, noble but spoiled Tommy Merton and his poor but virtuous friend Harry Sandford.

[7] He attempted to put his theory to the test with a colt on 28 September 1789 at Barehill, Berkshire, but was thrown from the horse, and died almost instantly.