He was further aided by an advantageous marriage on 5 October 1749 to Elizabeth, daughter of Nicholas Jeffreys of the Priory, Brecknock, by whom he had a son John Jeffreys, his successor in title and estates, and four daughters, of whom the eldest, Frances, married Robert Stewart, 1st Marquess of Londonderry on 7 June 1775.
The first case which brought him prominently into notice and gave him assurance of ultimate success was the government prosecution, in 1752, of a bookseller, William Owen. Owen had published a book The Case of Alexander Murray, Esq; in an Appeal to the people of Great Britain which the House of Commons had, by resolution of the House, condemned as "an impudent, malicious, scandalous and seditious libel". The author had left the country so the weight of the government's censure fell on Owen. Pratt appeared in Owen's defence and his novel argument was that it was not the sole role of the jury to determine the fact of publication but that it was further their right to assess the intent of a libel. In his summing up, the judge, Lord Chief Justice Sir William Lee directed the jury to find Owen guilty as publication was proved and the intent of the contents was a question of law for the judge, not a question of fact for the jury. The jury disagreed and acquitted Owen. Pratt was appointed King's Counsel in 1755.
In July 1757, Pitt formed a coalition government with Newcastle and insisted on Pratt's appointment as Attorney-General. Pratt was preferred over Solicitor General Charles Yorke. Yorke was the son of Lord Hardwick, a political ally of Newcastle who, as Lord Chancellor had obstructed Pratt's career in favour of his own son. Though this led to an uncomfortable relationship between the two law officers of the Crown, it led to the landmark Pratt-Yorke opinion of 24 December 1757 whereby the pair distinguished overseas territories acquired by conquest from those acquired by private treaty. They asserted that, while the Crown of Great Britain enjoyed sovereignty over both, only the property of the former was vested in the Crown. Though the original opinion related to the British East India Company, it came to be applied elsewhere in the developing British Empire.
The same year he entered the House of Commons as Member of Parliament (MP) for the borough of Downton in Wiltshire. He sat in Parliament for four years, but did not distinguish himself as a debater. He introduced the Habeas corpus Amendment Bill of 1758, which was intended to extend the writ of Habeas corpus from criminal law to civil and political cases. Despite Pitt's support, the Bill fell in the House of Lords. At the same time, his professional practice increased, particularly his Chancery practice which made him financially secure and enabled him to purchase the Camden Place estate in Kent.
As Attorney-General, Pratt prosecuted Florence Hensey, an Irishman who had spied for France, and John Shebbeare (1709-1788), a violent party writer of the day. Shebbeare had published a libel against the government contained in his Letters to the People of England, which were published in the years 1756-1758. As evidence of Pratt's moderation in a period of passionate party warfare and frequent state trials, it is notable that this was the only official prosecution for libel that he started and that he maintained his earlier insistence that the decision lay with the jury. He led for the Crown in the prosecution of Laurence Shirley, 4th Earl Ferrers for the murder of a servant, a case that shocked European society.
The Common Pleas was not an obvious forum for a jurist with constitutional interest, dealing as it did principally with disputes between private parties. However, on 30 April 1763, Member of Parliament John Wilkes was arrested under a general warrant for alleged seditious libel in issue No.45 of The North Briton. Pratt freed Wilkes holding that parliamentary privilege gave him immunity from arrest on such a charge. The decision earned Pratt some favour with the radical faction in London and seems to have spurred him, over the summer of that year to encourage juries to award disproportionate and excessive damages to printers unlawfully arrested over the same matter. Wilkes was awarded £1,000 (£127,000 at 2003 prices) and Pratt condemned the use of general warrants for entry and search. Pratt pronounced with decisive and almost passionate energy against their legality, thus giving voice to the strong feeling of the nation and winning for himself an extraordinary degree of popularity as one of the maintainers of English civil liberties. Honours fell thick upon him in the form of addresses from the City of London and many large towns, and of presentations of freedom from various corporate bodies.
In 1762, the home of John Entick had been raided by officers of the Crown, searching for evidence of sedition. In the case of Entick v. Carrington (1765), Pratt held that the raids were unlawful as they were without authority in statute or in common law.
Pitt and his followers had, after their initial opposition, come to support the Declaratory Act of 1766 which asserted Great Britain's sovereignty over the American colonies. Further, continued unrest in America, stemming from Townshend's 1767 taxation scheme, brought a robust response from Pitt and Camden was his spokesman in the Lords. However, towards the end of 1767, Pitt fell ill and the Duke of Grafton stepped in as caretaker. Camden became indecisive in his own political role, writing to Grafton on 4 October 1768:
Pitt resigned on 14 October and Camden, who continued to sit in the cabinet as Lord Chancellor, now took up a position of uncompromising hostility to the governments of Grafton and Lord North on America and on Wilkes. Camden opposed Lord Hillsborough's confrontational approach to the Americas, favouring conciliation and working on the development on reformed tax proposals. Camden personally promised the colonies that no further taxes would be levied and voted in the cabinet minority who sought to repeal the tea duty.
He continued steadfastly to oppose the taxation of the American colonists, and signed, in 1778, the protest of the Lords in favour of an address to the King on the subject of the manifesto of the commissioners to America. In 1782 he was appointed Lord President of the Council under the Rockingham administration, but retired in the following year. Within a few months he was reinstated in this office under the Pitt administration, and held it till his death.
Lord Camden was a strenuous opponent of Fox's East India Bill, took an animated part in the debates on important public matters till within two years of his death, introduced in 1786 the scheme of a regency on occasion of the king's insanity, and to the last zealously defended his early views on the functions of juries, especially of their right to decide on all questions of libel. He was raised to the dignity of Earl Camden in May 1786, and was at the same time created Viscount Bayham. In 1788 he obtained an Act of Parliament granting permission to develop some fields he owned just to the north of London. This was the beginning of Camden Town.
The Earl Camden died in London on 18 April 1794. His remains were interred in Seal church in Kent.
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