In May of 1825, writing to Henry Lee, Thomas Jefferson set forth the classic sources of the principles of the Declaration of Independence, including human equality, self-government, and the individual rights of “Life, Liberty and the Pursuit of Happiness.” He wrote:
"This was the object of the Declaration of Independence. Not to find out new principles, or new arguments, never before thought of, not merely to say things which had never been said before; but to place before mankind the common sense of the subject … it was intended to be an expression of the American mind … All its authority rests then on the harmonizing sentiments of the day, whether expressed in conversation, in letters, printed essays, or in the elementary books of public right, as Aristotle, Cicero, Locke, Sidney, etc." [Images above].
While each of his named political philosophers, Aristotle, Cicero, Locke, and Sidney, were advocates for “public right[s],” each of them were also moralists, and Jefferson was intimately familiar with all of their writings. As taught in his Nicomachean Ethics, Aristotle defines virtue as a disposition to behave in the right manner, and vice as deficiencies or excesses in character. In addition to the nature of the virtues and vices involved in moral evaluation, he addresses the methods of achieving happiness in human life. Cicero’s On Duties analyzes what is “honorable” (honestas) and what is “beneficial” (or advantageous), and what is honorable can also be called “moral,” “virtuous,” “ethical,” or “noble.” The main components of noble behavior according to Cicero are virtue and duty, and he concludes that moral worth is the only good and that virtue is sufficient for happiness. In his 1690 Essay Concerning Human Understanding, Locke states that, “the necessity of pursuing happiness [is] the foundation of liberty” and that, “Morality is the proper Science, and Business of Mankind in general.” We must also remember that near the end of his life, Aristotle had to flee Athens, Cicero was proscribed an enemy of Rome and assassinated, and Locke fled England to Holland in order to escape King Charles II.
Yet, while Locke was a member of Jefferson’s triumvirate of the three greatest minds (along with Bacon and Newton), he reserved his highest political praise for Algernon Sidney. In addition to citing Sidney’s writings as a source for the principles of The Declaration, he endorsed Sidney's Discourses Concerning Government as “a rich treasure of republican principles” and “probably the best elementary book of the principles of government, as founded in natural right which has ever been published in any language.” And, Jefferson, together with James Madison, stated that “the general principles of liberty and the rights of man, in nature and society” were to be found in Locke's Second Treatise on Government and in Sidney's Discourses Concerning Government. So, while much less known than Locke in our day, Jefferson actually gave equal weight to Sidney’s Discourses alongside Locke in his proscribed course on the Constitution at the University of Virginia.
What makes Sidney unique as a source of Jefferson’s philosophy of virtue and happiness is that, unlike Locke who focused more on property rights, Sidney wrote profusely concerning the connection between liberty and virtue. Sidney stated, “The principle of liberty in which God created us …includes the chief advantages of the life we enjoy, as well as the greatest helps towards felicity, that is the end of our hopes in the other.” In other words, “life, liberty, and happiness” are mutually dependent. Jefferson also quoted Sidney in his Commonplace Book, recording in his own hand, “If vice and corruption prevail, liberty cannot subsist; but if virtue have the advantage, arbitrary power cannot be established.” Much less fortunate than Locke, Sidney was arrested, accused with the crime of high treason against King Charles II and was executed on December 7, 1683. Known in the American colonies as the “true martyr of liberty” the influence of Sidney on Jefferson and the principles of the Declaration of Independence cannot be discounted.
 Thomas Jefferson to Henry Lee, May 8, 1825, ME 16:118-19.
 Thomas Jefferson to John Trumbull, 18 January 1789, The Papers of Thomas Jefferson, Julian P. Boyd, ed. (Princeton: Princeton University Press, 1950), 14:467-68.
 Minutes of the Board of Visitors, March 4, 1825, ME 19:460-61 (cited as “Minutes”). http://www.liberty1.org/UVA1825.pdf
 Algernon Sidney, Discourses Concerning Government (London: A. Millar, London, 1751)(cited as “Discourses”), I:2:5.
 Discourses, II:30:241-242.
 c.f. Algernon Sidney, Discourses Concerning Government, Thomas G. West, ed. (Liberty Fund, Inc., Indianapolis, 1996), Introduction, xvi.
 Thomas Jefferson, Report for the Commissioners for the University of Virginia, August 4, 1818 (Special Collections Department, University of Virginia Library).