http://www.sciencefriday.com/segments/carl-sagan-science-is-a-way-of-thinking/
Carl Sagan: 'Science Is a Way of Thinking', Science Friday interview from May 1996
27 December 2013

“A central lesson of science is that to understand complex issues (or even simple ones), we must try to free our minds of dogma and to guarantee the freedom to publish, to contradict, and to experiment. Arguments from authority are unacceptable.”

Carl Sagan

Source: Billions and Billions: Thoughts on Life and Death at the Brink of the Millenium (1997), Chapter 14, "The Common Enemy".

“Relativity does set limits on what humans can ultimately do. But the universe is not required to be in perfect harmony with human aspirations.”

Carl Sagan book Cosmos

Source: Cosmos (1980), p. 201

“Humans may crave absolute certainty; they may aspire to it; they may pretend, as partisans of certain religions do, to have attained it. But the history of science — by far the most successful claim to knowledge accessible to humans — teaches that the most we can hope for is successive improvement in our understanding, learning from our mistakes, an asymptotic approach to the Universe, but with the proviso that absolute certainty will always elude us.”

Carl Sagan

Source: The Demon-Haunted World : Science as a Candle in the Dark (1995), Ch. 2 : Science and Hope, p. 28

“It took the Church until 1832 to remove Galileo's work from its list of books which Catholics were forbidden to read at the risk of dire punishment of their immortal souls.”

Carl Sagan book Pale Blue Dot

Source: Pale Blue Dot: A Vision of the Human Future in Space (1994), p. 43

“Appeal to ignorance — the claim that whatever has not been proved false must be true, and vice versa (e. g. There is no compelling evidence that UFOs are not visiting the Earth; therefore UFOs exist — and there is intelligent life elsewhere in the universe. Or: There may be seventy kazillion other worlds, but not one is known to have the moral advancement of the Earth, so we're still central to the Universe.) This impatience with ambiguity can be criticized in the phrase: absence of evidence is not evidence of absence.”

Carl Sagan

Referring to an aphorism of Martin Rees. (see Misattributed below)
Source: The Demon-Haunted World : Science as a Candle in the Dark (1995), Ch. 12 : The Fine Art of Baloney Detection, p. 221

“Ann Druyan suggests an experiment: Look back again at the pale blue dot of the preceding chapter. Take a good long look at it. Stare at the dot for any length of time and then try to convince yourself that God created the whole Universe for one of the 10 million or so species of life that inhabit that speck of dust. Now take it a step further: Imagine that everything was made just for a single shade of that species, or gender, or ethnic or religious subdivision. If this doesn't strike you as unlikely, pick another dot. Imagine it to be inhabited by a different form of intelligent life. They, too, cherish the notion of a God who has created everything for their benefit. How seriously do you take their claim?”

Carl Sagan book Pale Blue Dot

Source: Pale Blue Dot: A Vision of the Human Future in Space (1994), p. 11

“That it will never come again
Is what makes life so sweet.”

Carl Sagan book Contact

Quoting Emily Dickinson; The Poems of Emily Dickinson http://books.google.gr/books?id=LoH2SXEnnoEC&dq=, 3:1171, no. 1741 <br class="br">Source: Contact (1985), Chapter 22 (p. 393)

“We humans appear on the cosmic calendar so recently that our recorded history occupies only the last few seconds of the last minute of December 31st.”

Carl Sagan

57 min 0 sec
Cosmos: A Personal Voyage (1990 Update), The Shores of the Cosmic Ocean [Episode 1]

“I never said it. Honest. Oh, I said there are maybe 100 billion galaxies and 10 billion trillion stars. It's hard to talk about the Cosmos without using big numbers. I said 'billion' many times on the Cosmos television series, which was seen by a great many people. But I never said 'billions and billions.' For one thing, it's imprecise. How many billions are 'billions and billions'? A few billion? Twenty billion? A hundred billion? 'Billions and billions' is pretty vague… For a while, out of childish pique, I wouldn't utter the phrase, even when asked to. But I've gotten over that. So, for the record, here it goes: 'Billions and billions.”

Carl Sagan

List of misquotations
Billions and Billions: Thoughts on Life and Death at the Brink of the Millenium (1997)

“Our intelligence and our technology have given us the power to affect the climate. How will we use this power? Are we willing to tolerate ignorance and complacency in matters that affect the entire human family? Do we value short-term advantages above the welfare of the Earth? Or will we think on longer time scales, with concern for our children and our grandchildren, to understand and protect the complex life-support systems of our planet? The Earth is a tiny and fragile world. It needs to be cherished.”

Carl Sagan book Cosmos

Source: Cosmos (1980), p. 103

“If we like them, they’re freedom fighters, she thought. If we don’t like them, they’re terrorists. In the unlikely case we can’t make up our minds, they’re temporarily only guerrillas.”

Carl Sagan book Contact

Source: Contact (1985), Chapter 3 (p. 55)