From Magna Carta To Edward Bushell
The power of the jury to judge the justice of the law and to hold laws invalid by a finding of "not guilty" for any law a juror felt was unjust or oppressive, dates back to the Magna Carta, in 1215. At the time of the Magna Carta, King John could pass any law any time he pleased. Judges and executive officers, appointed and removed at his whim, were little more than servants of the King. The oppression became so great that the nation rose up against the ruler, and the barons of England compelled their king to pledge that he would not punish a freeman for a violation of the law without the consent of his peers.
King John violently protested when the Magna Carta was shown to him, and with a solemn oath protested, that "he would never grant such liberties as would make himself a slave." Afterwards, fearing seizure of his castle and the loss of his throne, he reluctantly signed the Magna Carta – thus placing the liberties of the people in their own safe-keeping. Echard's History of England, p. 106-107
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The Magna Carta was a great step forward in the control of tyrannical leaders. But its sole means of enforcement, the jury, was often met with hostility. By 1664 English juries were routinely being fined for acquitting defendants. Such was the case in the 1670 political trial of William Penn, who was charged with preaching Quakerism to an unlawful assembly. Four of the twelve jurors voted to acquit – and continued to acquit even after being imprisoned and starved for four days. Under such duress, most jurors paid the fines. However, one juror, Edward Bushell, refused to pay and brought his case before the Court of Common Pleas. As a result, Chief Justice Vaughan issued an historically-important ruling: that jurors could not be punished for their verdicts. Bushell's Case (1670) was one of the most important developments in the common-law history of the jury.
http://www.isil.org/resources/lit/history-jury-null.html