With a little bit of help from the Quotations Page:

The man who insists on seeing with perfect clearness before he decides, never decides. Henri-Frédéric Amiel

I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth. Umberto Eco (1932 – )

Mankind have a great aversion to intellectual labor; but even supposing knowledge to be easily attainable, more people would be content to be ignorant than would take even a little trouble to acquire it. Samuel Johnson (1709 – 1784), quoted in Boswell’s Life of Johnson

In science one tries to tell people, in such a way as to be understood by everyone, something that no one ever knew before. But in poetry, it’s the exact opposite. Paul Dirac (1902 – 1984)

My religion consists of a humble admiration of the illimitable superior spirit who reveals himself in the slight details we are able to perceive with our frail and feeble mind. Albert Einstein (1879 – 1955)

It is impossible to defeat an ignorant man in argument. William G. McAdoo (1863 – 1941)

The intelligent man finds almost everything ridiculous, the sensible man hardly anything. Johann Wolfgang von Goethe (1749 – 1832)

If you want to tell people the truth, make them laugh, otherwise they’ll kill you. Oscar Wilde (1854 – 1900)

All you need in this life is ignorance and confidence; then success is sure. Mark Twain (1835 – 1910), Letter to Mrs Foote, Dec. 2, 1887

It is difficult to get a man to understand something when his job depends on not understanding it. Upton Sinclair (1878 – 1968)

Happiness is always a by-product. It is probably a matter of temperament, and for anything I know it may be glandular. But it is not something that can be demanded from life, and if you are not happy you had better stop worrying about it and see what treasures you can pluck from your own brand of unhappiness. Robertson Davies

When a thing is funny, search it carefully for a hidden truth. George Bernard Shaw (1856 – 1950)

Stupidity, madame.  Pure stupidity.  Samuel Johnson (1709 – 1784), (attributed, allegedly in response to a woman who asked him how he came to define “pastern” as “the knee of a horse” in his dictionary.)

Opportunity is missed by most people because it is dressed in overalls and looks like work. Thomas A. Edison (1847 – 1931)

Everyone has the obligation to ponder well his own specific traits of character. He must also regulate them adequately and not wonder whether someone else’s traits might suit him better. The more definitely his own a man’s character is, the better it fits him. Cicero (106 BC – 43 BC)

Thou art the Mars of malcontents. William Shakespeare (1564 – 1616), “The Merry Wives of Windsor”, Act 1 scene 3

A fanatic is one who can’t change his mind and won’t change the subject. Sir Winston Churchill (1874 – 1965)

Men occasionally stumble over the truth, but most of them pick themselves up and hurry off as if nothing ever happened. Sir Winston Churchill (1874 – 1965)

Power tends to corrupt, and absolute power corrupts absolutely. Lord Acton, Letter to Bishop Mandell Creighton, 1887

Good ideas are not adopted automatically. They must be driven into practice with courageous patience. Hyman Rickover (1900 – 1986)

The best way to convince a fool that he is wrong is to let him have his own way. Josh Billings (1818 – 1885)

To punish me for my contempt for authority, fate made me an authority myself. Albert Einstein (1879 – 1955)

Repetition does not transform a lie into a truth. Franklin D. Roosevelt (1882 – 1945), radio address, October 26, 1939

Wit is so shining a quality that everybody admires it; most people aim at it, all people fear it, and few love it unless in themselves. Lord Chesterfield (1694 – 1773), letter to his godson, December 18, 1765

He can compress the most words into the smallest ideas of any man I ever met. Abraham Lincoln (1809 – 1865)

Your manuscript is both good and original, but the part that is good is not original and the part that is original is not good. Samuel Johnson (1709 – 1784), (attributed)

Convinced myself, I seek not to convince. Edgar Allan Poe (1809 – 1849), Berenice

If you are out to describe the truth, leave elegance to the tailor. Albert Einstein (1879 – 1955)

Life does not cease to be funny when people die any more than it ceases to be serious when people laugh. George Bernard Shaw (1856 – 1950)

Common sense is the collection of prejudices acquired by age eighteen. Albert Einstein (1879 – 1955), (attributed)

Whatever you are, be a good one. Abraham Lincoln (1809 – 1865)

You are not here merely to make a living. You are here to enable the world to live more amply, with greater vision, and with a finer spirit of hope and achievement. You are here to enrich the world. You impoverish yourself if you forget this errand. Woodrow Wilson (1856 – 1924)

Do what thy manhood bids thee do, from none but self expect applause; He noblest lives and noblest dies who makes and keeps his self-made laws. Sir Richard Francis Burton (1821 – 1890)

A sense of duty is useful in work, but offensive in personal relations. People wish to be liked, not be endured with patient resignation. Bertrand Russell (1872 – 1970), Conquest of Happiness (1930) ch. 10

If two men agree on everything, you may be sure that one of them is doing the thinking. Lyndon B. Johnson (1908 – 1973)

No one means all he says, and yet very few say all they mean, for words are slippery and thought is viscous. Henry Adams (1838 – 1918)

When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong. Arthur C. Clarke (1917 – ), Clarke’s first law

In the end, we will remember not the words of our enemies, but the silence of our friends. Martin Luther King Jr. (1929 – 1968)

Most men pursue pleasure with such breathless haste that they hurry past it. Soren Kierkegaard (1813 – 1855)

None are so busy as the fool and knave. John Dryden (1631 – 1700), The Medal, 1682

Doubt is not a pleasant condition, but certainty is absurd. Voltaire (1694 – 1778)

Even on the highest throne in the world, we are still sitting on our ass. Michel de Montaigne (1533 – 1592), ‘De l’experience,’ 1580-88

A good man would prefer to be defeated than to defeat injustice by evil means. Sallust (86 BC – 34 BC), ‘Jugurthine War,’ 41 B.C.

There is no nonsense so gross that society will not, at some time, make a doctrine of it and defend it with every weapon of communal stupidity. Robertson Davies

A person reveals his character by nothing so clearly as the joke he resents. Georg Christoph Lichtenberg (1742 – 1799)

Actions have consequences…first rule of life. And the second rule is this – you are the only one responsible for your own actions. Holly Lisle, Fire In The Mist, 1992

Our lives improve only when we take chances – and the first and most difficult risk we can take is to be honest with ourselves. Walter Anderson

Flattery is like cologne water, to be smelt of, not swallowed. Josh Billings (1818 – 1885)

Once a government is committed to the principle of silencing the voice of opposition, it has only one way to go, and that is down the path of increasingly repressive measures, until it becomes a source of terror to all its citizens and creates a country where everyone lives in fear. Harry S Truman (1884 – 1972), August 8, 1950

Facts do not cease to exist because they are ignored. Aldous Huxley (1894 – 1963), “Proper Studies”, 1927

There are people who, instead of listening to what is being said to them, are already listening to what they are going to say themselves. Albert Guinon (1863 – 1923)

When everyone is against you, it means that you are absolutely wrong– or absolutely right. Albert Guinon (1863 – 1923)

Nothing is more conducive to peace of mind than not having any opinions at all. Georg Christoph Lichtenberg (1742 – 1799)

The sad truth is that excellence makes people nervous. Shana Alexander

Talk sense to a fool and he calls you foolish. Euripides (484 BC – 406 BC)

I am not one of those who in expressing opinions confine themselves to facts. Mark Twain (1835 – 1910), Wearing White Clothes speech, 1907

The multitude of books is making us ignorant. Voltaire (1694 – 1778)

One machine can do the work of fifty ordinary men. No machine can do the work of one extraordinary man. Elbert Hubbard (1856 – 1915)

What you cannot enforce, do not command. Sophocles (496 BC – 406 BC)

Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices, but honestly and courageously uses his intelligence and fulfills the duty to express the results of his thought in clear form. Albert Einstein (1879 – 1955), quoted in New York Times, March 19, 1940

Only two things are infinite, the universe and human stupidity, and I’m not sure about the former. Albert Einstein (1879 – 1955)

He draweth out the thread of his verbosity finer than the staple of his argument. William Shakespeare (1564 – 1616), “Love’s Labour’s Lost”, Act 5 scene 1

Their understanding/Begins to swell and the approaching tide/Will shortly fill the reasonable shores/That now lie foul and muddy. William Shakespeare (1564 – 1616)

Sweet are the uses of adversity, which, like a toad, though ugly and venomous, wears yet a precious jewel in its head. William Shakespeare (1564 – 1616)

He who has injured thee was either stronger or weaker than thee. If weaker, spare him; if stronger, spare thyself. William Shakespeare (1564 – 1616)

A wretched soul, bruised with adversity,/We bid be quiet when we hear it cry; /But were we burdened with like weight of pain,/As much or more we should ourselves complain. William Shakespeare (1564 – 1616)

Small minds are much distressed by little things. Great minds see them all but are not upset by them. Francois de La Rochefoucauld (1613 – 1680)

If we had no faults of our own, we would not take so much pleasure in noticing those of others. Francois de La Rochefoucauld (1613 – 1680)

A military operation involves deception. Even though you are competent, appear to be incompetent. Though effective, appear to be ineffective. Sun-tzu (~400 BC), The Art of War. Strategic Assessments

Be extremely subtle, even to the point of formlessness. Be extremely mysterious, even to the point of soundlessness. Thereby you can be the director of the opponent’s fate. Sun-tzu (~400 BC), The Art of War. Emptiness and Fullness

Victorious warriors win first and then go to war, while defeated warriors go to war first and then seek to win. Sun-tzu (~400 BC), The Art of War. Strategic Assessments

The secret of success is constancy of purpose. Benjamin Disraeli (1804 – 1881)

As a general rule the most successful man in life is the man who has the best information. Benjamin Disraeli (1804 – 1881)

A precedent embalms a principle. Benjamin Disraeli (1804 – 1881)

The most dangerous strategy is to jump a chasm in two leaps. Benjamin Disraeli (1804 – 1881)

An object in possession seldom retains the same charm that it had in pursuit. Pliny the Younger (62 AD – 114 AD), Letters

However often you may have done them a favour, if you once refuse they forget everything except your refusal. Pliny the Younger (62 AD – 114 AD)

Regard your good name as the richest jewel you can possibly be possessed of – for credit is like fire; when once you have kindled it you may easily preserve it, but if you once extinguish it, you will find it an arduous task to rekindle it again. The way to gain a good reputation is to endeavor to be what you desire to appear. Socrates (469 BC – 399 BC)

Wise men talk because they have something to say; fools, because they have to say something. Plato (427 BC – 347 BC)

No human thing is of serious importance. Plato (427 BC – 347 BC)

It is the mark of an educated mind to be able to entertain a thought without accepting it. Aristotle (384 BC – 322 BC)

Humor is the only test of gravity, and gravity of humor; for a subject which will not bear raillery is suspicious, and a jest which will not bear serious examination is false wit. Aristotle (384 BC – 322 BC)

Do not fear to be eccentric in opinion, for every opinion now accepted was once eccentric. Bertrand Russell (1872 – 1970)

The time you enjoy wasting is not wasted time. Bertrand Russell (1872 – 1970)

So far as I can remember, there is not one word in the Gospels in praise of intelligence. Bertrand Russell (1872 – 1970)

The mind is its own place, and in itself can make a Heaven of Hell, a Hell of Heaven. John Milton (1608 – 1674)

One doesn’t have a sense of humor. It has you. Larry Gelbart

Everything is vague to a degree you do not realize till you have tried to make it precise. Bertrand Russell (1872 – 1970)

Never interrupt your enemy when he is making a mistake. Napoleon Bonaparte (1769 – 1821)

The fact that an opinion has been widely held is no evidence whatever that it is not utterly absurd; indeed in view of the silliness of the majority of mankind, a widespread belief is more likely to be foolish than sensible. Bertrand Russell (1872 – 1970), Marriage and Morals (1929) ch. 5

God is a comedian playing to an audience too afraid to laugh. Voltaire (1694 – 1778)

Seriousness is the only refuge of the shallow. Oscar Wilde (1854 – 1900)

A man is not idle because he is absorbed in thought. There is a visible labor and there is an invisible labor. Victor Hugo (1802 – 1885)

In theory, there is no difference between theory and practice; In practice, there is. Chuck Reid

There is always a well-known solution to every human problem–neat, plausible, and wrong. H. L. Mencken (1880 – 1956), Prejudices: Second Series, 1920

It is hard to believe that a man is telling the truth when you know that you would lie if you were in his place. H. L. Mencken (1880 – 1956)

Every decent man is ashamed of the government he lives under. H. L. Mencken (1880 – 1956)

There are three classes of intellects: one which comprehends by itself; another which appreciates what others comprehend; and a third which neither comprehends by itself nor by the showing of others; the first is the most excellent, the second is good, and the third is useless. Niccolo Machiavelli (1469 – 1527), The Prince

The enemy is anybody who’s going to get you killed, no matter which side he’s on. Joseph Heller (1923 – 1999), Catch 22

Men are generally idle, and ready to satisfy themselves, and intimidate the industry of others, by calling that impossible which is only difficult. Samuel Johnson (1709 – 1784), Life of Boerhaave