“Good humor is one of the preservatives of our peace and tranquility” 

“If by religion we are to understand sectarian dogmas, in which no two of them agree, then your exclamation on that hypothesis is just, "that this would be the best of worlds if there were no religion in it.” 

“Our properties within our own territories [should not] be taxed or regulated by any power on earth but our own.”

“When injustice becomes law, resistance becomes duty.”

“I am increasingly persuaded that the earth belongs exclusively to the living and that one generation has no more right to bind another to it's laws and judgments than one independent nation has the right to command another.”

“He has endeavoured to prevent the population of these States; for that purpose obstructing the Laws of Naturalization of Foreigners; refusing to pass others to encourage their migration hither, and raising the conditions of new Appropriations of Lands.”

“Follow truth wherever it may lead you.”

“I was bold in the pursuit of knowledge, never fearing to follow truth and reason to whatever results they led, and bearding every authority which stood in their way. ”

“It is more dangerous that even a guilty person should be punished without the forms of law than that he should escape.”

“There exists indeed an opposition to it [building of UVA, Jefferson's secular college] by the friends of William and Mary, which is not strong. The most restive is that of the priests of the different religious sects, who dread the advance of science as witches do the approach of day-light; and scowl on it the fatal harbinger announcing the subversion of the duperies on which they live. In this the Presbyterian clergy take the lead. The tocsin is sounded in all their pulpits, and the first alarm denounced is against the particular creed of Doctr. Cooper; and as impudently denounced as if they really knew what it is.

“To your request of my opinion of the manner in which a newspaper should be conducted, so as to be most useful, I should answer, ‘by restraining it to true facts & sound principles only.’ Yet I fear such a paper would find few subscribers. It is a melancholy truth, that a suppression of the press could not more compleatly deprive the nation of its benefits, than is done by its abandoned prostitution to falsehood. Nothing can now be believed which is seen in a newspaper. Truth itself becomes suspicious by being put into that polluted vehicle. The real extent of this state of misinformation is known only to those who are in situations to confront facts within their knolege with the lies of the day. I really look with commiseration over the great body of my fellow citizens, who, reading newspapers, live & die in the belief, that they have known something of what has been passing in the world in their time; whereas the accounts they have read in newspapers are just as true a history of any other period of the world as of the present, except that the real names of the day are affixed to their fables. General facts may indeed be collected from them, such as that Europe is now at war, that Bonaparte has been a successful warrior, that he has subjected a great portion of Europe to his will, &c., &c.; but no details can be relied on. I will add, that the man who never looks into a newspaper is better informed than he who reads them; inasmuch as he who knows nothing is nearer to truth than he whose mind is filled with falsehoods & errors. He who reads nothing will still learn the great facts, and the details are all false.” —Letter to John Norvell, 14 June 1807 [Works 10:417--18]” 

“The object most interesting to me for the residue of my life, will be to see you both developing daily those principles of virtue and goodness which will make you valuable to others and happy in yourselves, and acquiring those talents and that degree of science which will guard you at all times against ennui, the most dangerous poison of life. A mind always employed is always happy. This is the true secret, the grand recipe for felicity....In a world which furnishes so many employments which are useful, and so many which are amusing, it is our own fault if we ever know what ennui is...”

“When angry, count ten before you speak; if very angry, an hundred.”

“I consider him [Alexander von Humboldt] the most important scientist whom I have met.”

“The majority, oppressing an individual, is guilty of a crime, abuses its strength, and by acting on the law of the strongest breaks up the foundations of society.”