...and trump card of the people against tyranny.
The following is from Lysander Spooner's "Essay on the Trial by Jury" (1852)
THE RIGHT OF JURIES TO JUDGE OF THE JUSTICE OF LAWS
FOR more than six hundred years that is, since Magna Carta, in 1215 there has been no clearer principle of English or American constitutional law, than that, in criminal cases, it is not only the right and duty of juries to judge what are the facts, what is the law, and what was the moral intent of the accused; but that it is also their right, and their primary and paramount duty, to judge of the justice of the law, and to hold all laws invalid, that are, in their opinion, unjust or oppressive, and all persons guiltless in violating, or resisting the execution of, such laws.
Unless such be the right and duty of jurors, it is plain that, instead of juries being a "palladium of liberty" a barrier against the tyranny and oppression of the government they are really mere tools in its hands, for carrying into execution any injustice and oppression it may desire to have executed.
But for their right to judge of the law, and the justice of the law, juries would be no protection to an accused person, even as to matters of fact; for, if the government can dictate to a jury any law whatever, in a criminal case, it can certainly dictate to them the laws of evidence. That is, it can dictate what evidence is admissible, and what inadmissible, and also what force or weight is to be given to the evidence admitted. And if the government can thus dictate to a jury the laws of evidence, it can not only make it necessary for them to convict on a partial exhibition of the evidence rightfully pertaining to the case, but it can even require them to convict on any evidence whatever that it pleases to offer them.
That the rights and duties of jurors must necessarily be such as are here claimed for them, will be evident when it is considered what the trial by jury is, and what is its object.
"The trial by jury," then, is a "trial by the country" that is, by the people as distinguished from a trial by the government.
The object of this trial "by the country," or by the people, in preference to a trial by the government, is to guard against every species of oppression by the government. In order to effect this end, it is indispensable that the people, or "the country," judge of and determine their own liberties against the government; instead of the government's judging of and determining its own powers over the people. How is it possible that juries can do anything to protect the liberties of the people against the government, if they are not allowed to determine what those liberties are?
Any government, that is its own judge of, and determines authoritatively for the people, what are its own powers over the people, is an absolute government of course. It has all the powers that it chooses to exercise. There is no other or at least no more accurate definition of a despotism than this.
On the other hand, any people, that judge of, and determine authoritatively for the government, what are their own liberties against the government, of course retain all the liberties they wish to enjoy. And this is freedom. At least, it is freedom to them; because, although it may be theoretically imperfect, it, nevertheless, corresponds to their highest notions of freedom.