About this series. The Critical Skills Blog is about critical skills and how to apply them. This three-part series examines critical thinking through two distinguished men who practiced it: Thomas Jefferson, a Founding Father who wanted to be a scientist, and Carl Sagan, a renowned scientist who tried to teach the public to think like one.

Article 001 tests Jefferson against his own standard of evidence, 002 opens Sagan’s toolkit for thinking, and 003 updates that toolkit for an age of algorithms and artificial intelligence.

This is 001.

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In an interview that aired on May 27, 1996, less than seven months before his death, Carl Sagan talked with journalist Charlie Rose about his new book, The Demon-Haunted World: Science as a Candle in the Dark.

The conversation turned to Thomas Jefferson. Jefferson, Sagan said, considered himself a scientist at heart, a man who “would have loved to have been a scientist.” The crises of his era called him into politics instead. Sagan then offered his own reading of the bargain: Jefferson’s political work helped make it possible for later generations to become scientists.

Moments earlier, Sagan had made a broader point. Rights written into a constitution depend on citizens who are educated and who practice their skepticism, and Jefferson, he noted, had laid great stress on exactly that.

Those two observations frame this series. Sagan gave the modern statement of critical thinking: withhold belief until the evidence compels it. If Jefferson was the scientist at heart Sagan described, that standard is the fair one to judge him by, in science and also in religion, in education, and in the politics that claimed his career. His letters show how far he met it.

Four Confessions

In June 1964 I visited Monticello with a Naval Academy classmate. I was surprised that there were so few people there. It was late morning, and we basically had the run of the place. I stood in Jefferson’s study, closed my eyes, and imagined him sitting there.

His letters tell us what he was thinking. He described his love of science in at least four of them, to three correspondents, across twenty-six years.

In 1791, as Secretary of State, he answered the Kentucky judge Harry Innes, who had sent news of natural history and promised political news in a second letter still on its way. “The first is my passion,” Jefferson wrote, “the last my duty, and therefore both desireable.”

In June 1807, writing to the Philadelphia anatomist Caspar Wistar about his grandson’s education, President Jefferson welcomed a moment’s escape from “the dry & dreary waste of politics” and a chance to “indulge in the rich fields of nature, where alone I should have served as a volunteer, if left to my natural inclinations & partialities.”

On March 2, 1809, two days before he left the presidency, he wrote to the French economist Pierre Samuel du Pont de Nemours:

Nature intended me for the tranquill pursuits of science, by rendering them my supreme delight. But the enormities of the times in which I have lived, have forced me to take a part in resisting them, and to commit myself on the boisterous ocean of political passions.

In 1817, long retired, he told Wistar he had nothing to offer the American Philosophical Society’s next Transactions, because public life had consumed the years for such work:

The truth is that I have been drawn by the history of the times from Physical & mathematical sciences, which were my passion, to those of politics & government towards which I had naturally no inclination.

A single line in a farewell letter might be a graceful pose. Repeated in private across a quarter century, the preference is harder to dismiss, though repetition proves only that it endured.

The historian Jon Meacham presses a sharper caution: Jefferson was powerfully drawn to politics, spent some forty years seeking and exercising power, and worked to shape how posterity would remember him. A man can sincerely love one calling while seeking, and enjoying, another.

The Other Presidency

Jefferson’s conduct backed his confessions. The American Philosophical Society, founded by Benjamin Franklin, elected him a member in 1780 and its president in January 1797, succeeding Franklin and the astronomer David Rittenhouse. In his formal acceptance, Jefferson called the election “the most flattering incident of my life.” He held the office through his vice presidency and both presidential terms. He resigned in November 1814; the Society, which had refused earlier offers, accepted in January 1815.

The Society’s historian Patrick Spero reports that in 1798, while Vice President, Jefferson attended every meeting held while he was in Philadelphia, and judges that he likely gave the Society more attention than the vice presidency. On March 10, 1797, Jefferson presented a paper on fossil bones he named the megalonyx, which Spero calls a founding moment for American vertebrate paleontology. In 1807 he arranged for William Clark to dig at Big Bone Lick, Kentucky, “at my expence, not that of the society.”

Before Meriwether Lewis left for the West in 1803, Jefferson sent him to Society members for training in astronomy, botany, anatomy, and medicine. In 1805 a French agricultural society gave him a gold medal for a plow moldboard designed on mathematical principles. His science also served his politics. Spero shows how his fossil work answered European claims that American nature was degenerate, and how western exploration served his vision of an expanding agrarian republic.

The Scientist’s Discipline

In 1787, responding to reports of ancient earthworks in the West, Jefferson warned his friend Charles Thomson against theorizing too soon:

The moment a person forms a theory, his imagination sees in every object only the traits which favor that theory.

It is a precise description of what psychologists now call confirmation bias, written long before the term existed.

In Notes on the State of Virginia, answering the French naturalist Buffon, he separated measured weights from estimates and offered his figures “to justify a suspension of opinion until we are better informed.” In 1807 he called for reform in the speculative side of medicine: “an abandonment of hypothesis for sober facts, the first degree of value set on clinical observation, and the lowest on visionary theories.”

After a solar eclipse in September 1811, he would not let his timings of the first and last contacts shape the result, because his party had started watching late, misled by a slow clock: “I have no confidence in the times of the 1st & ultimate contacts.” He relied instead on the moments the ring of sunlight formed and broke. When the mathematician William Lambert sent him the longitude calculations, Jefferson admitted that on his own “most probably I should have foundered by the way,” though he still questioned which observations Lambert had used.

The same 1787 letter that warns against premature theory leans on what was “generally agreed” to assert that “rock grows, and it seems that it grows in layers in every direction, as the branches of trees grow in all directions.” Consensus and analogy stand in for evidence there, producing exactly the premature theory he had just warned against. The 1807 medical letter still endorses bleeding and purging. He applied his method unevenly, and it ran ahead of his knowledge. The second is the condition of every working scientist. The first is the failure the method exists to catch.

Faith Before Reason’s Tribunal

Jefferson applied the same discipline to religion. Here the comparison with Sagan is most tempting, and most in need of testing.

In a letter dated August 10, 1787, he advised his nephew Peter Carr, then a student, on how to study religion:

Fix reason firmly in her seat, and call to her tribunal every fact, every opinion. Question with boldness even the existence of a god; because, if there be one, he must more approve the homage of reason, than that of blindfolded fear.

He told Carr: “Read the bible then, as you would read Livy or Tacitus.” Ordinary events could rest on a writer’s testimony; claims that contradicted the laws of nature required stronger evidence. He tested Joshua’s account of the sun standing still against astronomy: a spinning earth stopped short would have flattened “animals, trees, buildings.” Two centuries later, Sagan drew the same line on Rose’s program. He honored religion’s ethics, history, and compassion, and he objected when religion presented its accounts of the physical world as scientific knowledge.

Jefferson acted on the principle. By 1804, as President, he had cut passages from the Gospels to assemble a private digest of Jesus’s moral teachings. Around 1820 he made a fuller version in four languages, The Life and Morals of Jesus of Nazareth, keeping the teachings and setting the miracles aside. When ancient historians report “calves speaking” or “statues sweating blood,” he explained that year, readers reject such things as fables. He also rejected passages he judged incompatible with Jesus’s established character, which let his own portrait of Jesus decide what was authentic. That invites circular reasoning, as any careful reader can see.

Here lies Jefferson’s secret, with one distinction. His love of science was public. He wrote publicly about religion too, defending religious liberty in the Virginia statute and in Notes, where he wrote of slavery, “I tremble for my country when I reflect that God is just.” What he kept from the public was the particulars of his own creed, which he shared only with chosen friends. In the very letter that sent Benjamin Rush his outline of Jesus’s teachings in 1803, he said he was “averse to the communication of my religious tenets to the public.” He told a biographer in 1816: “say nothing of my religion. it is known to my god and myself alone.” The outline had already appeared in a London magazine, but because that magazine was scarcely known in America, he wrote in 1820, “the Syllabus therefore is still a secret.”

The Jefferson letter most important to me is a public one: his 1802 reply to the Danbury Baptist Association, with its “wall of separation between Church & State.” It matters to me for a family reason. Roger Williams, the founder of Rhode Island, was my great-great-great . . . grandfather. In 1644 he wrote of “a hedge or wall of separation between the garden of the church and the wilderness of the world.” Williams was more concerned about what the state would do to religion, as opposed to the other way around. Jefferson’s worry often ran the other way. In 1800 he swore “eternal hostility against every form of tyranny over the mind of man” while denouncing clergy who hoped for an established church.

I began this piece believing that Jefferson was an excellent example of practicing what Sagan was preaching, but his letters led me away from such simplicity.

The two men parted company on conclusions, and partly on method. Sagan applied his rule of withholding belief to religious claims as well. Jefferson, in an 1823 letter to John Adams that may never have been sent, held that the universe made it “impossible for the human mind not to percieve and feel a conviction of design,” and he counted the near-universal agreement of mankind as support. In 1787 he had told Carr that the belief of millions entitled a claim to inquiry and no more; Carr was to “neither believe nor reject any thing because any other person, or description of persons have rejected or believed it.” They shared a commitment to questioning authority; their standards of evidence did not always coincide. Jefferson’s bill for religious freedom explains why open disagreement is the right outcome: “truth is great and will prevail if left to herself.”

Spreading the Light

If reason was the method, education was the means of spreading it. From Paris in 1786 he urged his old teacher George Wythe, “Preach, my dear Sir, a crusade against ignorance,” and called the bill for spreading knowledge among the people “by far the most important bill in our whole code.”

His reasons were political as well as intellectual. “If a nation expects to be ignorant & free, in a state of civilisation,” he wrote in 1816, “it expects what never was & never will be.”

In 1820, disputing the idea that judges should settle every constitutional question, he wrote that if the people seemed unready for that control, the remedy was “not to take it from them, but to inform their discretion by education.” Sagan made the same point: rights on paper depend on educated citizens who practice their skepticism.

This subject is important to me because I strongly believe that critical thinking and a solid base of education is the only way that a constitutional republic can survive. That belief has been strengthened through my podcast series of the Federalist and Anti-Federalist papers, Making a Great America, as well as the letters exchanged between Thomas Jefferson and his two ex-president friends, John Adams and James Madison.

The University of Virginia was meant to be the capstone. “This institution will be based on the illimitable freedom of the human mind,” he wrote in 1820. “For here we are not afraid to follow truth wherever it may lead, nor to tolerate any error so long as reason is left free to combat it.”

He made an exception, and it matters. In private letters in 1825, including one to Joseph Cabell, he argued that the university’s governing board should prescribe the texts for teaching government, to guard students against “the diffusion of that poison.” Madison urged moderation and warned against compulsory conformity, though he too favored a politically orthodox professor. In 1826 Jefferson told Madison, “In selecting our Law-Professor, we must be rigorously attentive to his political principles.” The man who would follow truth wherever it led wanted to choose the road himself when the subject was politics.

The Tribunal Turned on Jefferson

Jefferson asked his nephew to call every opinion before reason’s tribunal. Fairness requires calling Jefferson there too, and on race the verdict is hard.

In Notes on the State of Virginia he advanced “as a suspicion only” the claim that Black people were inferior to whites “in the endowments both of body and mind.” He drew chiefly on people held in slavery, under conditions that made fair observation impossible. He dismissed the poet Phillis Wheatley rather than treat her work as counterevidence, and, after comparing enslaved people in ancient Rome, concluded that nature rather than condition produced the difference he claimed to see. In 1791 the free Black astronomer Benjamin Banneker sent him an almanac, with a letter challenging Jefferson directly on slavery. Jefferson replied warmly, “No body wishes more than I do to see such proofs as you exhibit,” and said he was sending the almanac to the Marquis de Condorcet in Paris. In February 1809 he told the French abolitionist Henri Grégoire that his doubts came from “the limited sphere of my own State,” adding: “whatever be their degree of talent it is no measure of their rights.”

That October, writing privately to the poet Joel Barlow, he dismissed Grégoire’s evidence, suggested that Banneker had help with his mathematics, and admitted sending Grégoire “a very soft answer.” He insisted he had expressed only a doubt, never a fixed opinion. Yet by the standard he had given Thomson, he saw only the traits that favored his theory when the theory was his own.

The same standard applies to slavery itself. In 1785 he told Peter Carr to “give up money, give up fame, give up science, give the earth itself & all it contains rather than do an immoral act.” The leisure that let Jefferson time eclipses and collect fossils rested on the labor of the people he enslaved at Monticello. When the young Virginian Edward Coles urged him in 1814 to lead the cause of emancipation, Jefferson called slavery “a mortal reproach to us,” then declined: “this enterprise is for the young.” He encouraged Coles to press within Virginia for gradual emancipation, paired with removing freed people from the country. But he urged Coles not to free the people he had inherited or leave the state, arguing that people raised in slavery could not yet care for themselves. Coles left anyway. On the Ohio River in 1819, bound for Illinois, he told a group of the people he had inherited that they were free. He later became governor of Illinois.

His contributions to science, religious liberty, and education stand. So does the lesson. Jefferson stated the standard of evidence with unusual clarity and failed it where his own interests and assumptions were most at stake. Sagan urged readers to turn their skepticism on their own arguments before presenting them to anyone else. Jefferson’s record shows why.

The Verdict He Wrote Himself

Near the end of his life, Jefferson designed his own grave marker and wrote its inscription, “& not a word more”: author of the Declaration of American Independence, author of the Statute of Virginia for religious freedom, and Father of the University of Virginia. He chose these because “by these, as testimonials that I have lived, I wish most to be remembered.” The presidency, the vice presidency, the State Department, and the governorship go unmentioned. Of all the evidence, this is what turned me most: he didn’t mention the political side of his life. Nothing about politics. Meacham reads the inscription as a deliberate effort to steer posterity’s judgment; it is also Jefferson’s own account of what mattered most. Two of the three achievements concern freedom of the mind: the freedom to believe and the freedom to learn.

Ten days before he died, declining an invitation to the fiftieth anniversary of independence, he wrote that “the general spread of the light of science” had laid open a plain truth: “the mass of mankind has not been born, with saddles on their backs, nor a favored few booted and spurred, ready to ride them legitimately, by the grace of god.”

Sagan’s reading was that Jefferson gave up the career he wanted so that later generations could have it.

That is an interpretation, and readers may weigh it. The letters establish something narrower and sturdier. Jefferson loved science, practiced its discipline in faith and education, wrote its standard down in plain words, and fell short of it where the cost to himself was greatest. His legacy holds all of it, the liberty and the slavery alike. The lesson this series takes from him is the standard itself, which binds every reader as fully as it bound him.

Coming in 002 Sagan

The purpose of this series is to examine critical thinking as two distinguished men practiced it. Jefferson wrote his standard of evidence into letters.

The next article focuses on Carl Sagan, who built the same standard into a toolkit anyone can carry: the “baloney detection kit” of The Demon-Haunted World, with nine tools for testing a claim and twenty traps that make weak arguments sound strong. It opens the kit, scores Jefferson against it, and then, as Sagan insisted, turns it on Sagan himself.

Charles Cranston Jett is an author, civic educator, and Professional Certified Coach based in Chicago. A graduate of the U.S. Naval Academy (Class of 1964) and Harvard Business School, he served aboard nuclear submarines during the Cold War. He is the author of six books, including Super Nuke!, hosts four podcasts, and writes across his Critical Skills Blog platform on history, leadership, and the health of the American republic. In his writing he uses AI tools for research, editing, and occasional image creation; the arguments, the voice, and the final judgment are his. He and his wife, Dr. Nancy Church, live at Water Tower Residences, where they co-host the Chicago Salons.

Sources

Sagan

Carl Sagan interview with Charlie Rose, May 27, 1996. Jefferson remarks at about 04:23 and 05:22–05:48 of the Sagan segment; religion remarks at about 07:18–10:30.
Carl Sagan, The Demon-Haunted World: Science as a Candle in the Dark (New York: Random House, copyright 1995; released February 1996).

Jefferson’s letters (Founders Online unless noted)

Related correspondence

Other primary documents

Notes on the State of Virginia, Queries VI, XIV, and XVIII (Documenting the American South)
A Bill for Establishing Religious Freedom, June 18, 1779
Jefferson’s instructions for his grave marker (Library of Congress)

Scholarship and reference

Patrick Spero, “The Other Presidency: Thomas Jefferson and the American Philosophical Society”, Proceedings of the American Philosophical Society 162, no. 4 (December 2018).
“American Philosophical Society”, Thomas Jefferson Encyclopedia, Monticello.
“The Life and Morals of Jesus of Nazareth” and “The Philosophy of Jesus of Nazareth”, Thomas Jefferson Encyclopedia, Monticello.
“Founding Farmer: Thomas Jefferson’s New and Improved Plow”, Library of Congress blog, December 10, 2025.
Jon Meacham, interview with David Daley, Salon, November 17, 2012.
Clarence W. Alvord, ed., Governor Edward Coles (Springfield: Illinois State Historical Library, 1920), including Coles’s account of the 1819 emancipation.
Lucia Stanton, “Those Who Labor for My Happiness”: Slavery at Thomas Jefferson’s Monticello (Charlottesville: University of Virginia Press, 2012).

Church and state

To the Danbury Baptist Association, January 1, 1802
To Benjamin Rush, September 23, 1800
Mark DeWolfe Howe, The Garden and the Wilderness: Religion and Government in American Constitutional History (Chicago: University of Chicago Press, 1965), on Roger Williams’s 1644 “hedge or wall of separation”

Quotations follow the Founders Online, Monticello, and Library of Congress transcriptions, including Jefferson’s spelling; initial capitals have been added where a quotation begins a sentence.

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