"Civilization cannot exist without new frontiers; it needs them both physically and spiritually. The physical need is obvious—new lands, new resources, new materials. The spiritual need is less apparent, but in the long run it is more important. We do not live by bread alone; we need adventure, variety, novelty, romance. As the psychologists have shown by their sensory-deprivation experiments, a man goes swiftly mad if he is isolated in a silent, darkened room, cut off completely from the external world. What is true of individuals is also true of societies; they, too, can become insane without sufficient stimulus."
—"Rocket to the Renaissance," Greetings, Carbon-Based Bipeds! Collected Essays, 1934-1998, 1999

"Two possibilities exist: Either we are alone in the Universe or we are not. Both are equally terrifying."
—quoted in Visions: How Science Will Revolutionize the Twenty-First Century, 1999, by Michio Kaku

"If we have learned one thing from the history of invention and discovery, it is that, in the long run—and often in the short one—the most daring prophecies seem laughably conservative."
—The Exploration of Space, 1951

"The greatest tragedy in mankind's entire history may be the hijacking of morality by religion."
—"Credo," Greetings, Carbon-Based Bipeds! Collected Essays, 1934-1998, 1999

"I can never look now at the Milky Way without wondering from which of those banked clouds of stars the emissaries are coming. If you will pardon so commonplace a simile, we have set off the fire alarm and have nothing to do but to wait. I do not think we will have to wait for long."
—The Sentinel, 1948

"All explorers are seeking something they have lost. It is seldom that they find it, and more seldom still that the attainment brings them greater happiness than the quest."
—The City and the Stars, 1956

"Space can be mapped and crossed and occupied without definable limit; but it can never be conquered. When our race has reached its ultimate achievements, and the stars themselves are scattered no more widely than the seed of Adam, even then we shall still be ants crawling on the face of the Earth. The ants have covered the world, but have they conquered it—for what do their countless colonies know of it, or of each other?"
—"We'll Never Conquer Space," Greetings, Carbon-Based Bipeds! Collected Essays, 1934-1998, 1999

"The only way of finding the limits of the possible is by going beyond them into the impossible."
—The Lost Worlds of 2001, 1972

"The fact that we have not yet found the slightest evidence for life—much less intelligence—beyond this Earth does not surprise or disappoint me in the least. Our technology must still be laughably primitive, we may be like jungle savages listening for the throbbing of tom-toms while the ether around them carries more words per second then they could utter in a lifetime."
—"Credo," Greetings, Carbon-Based Bipeds! Collected Essays, 1934-1998, 1999

"There is no reason to assume that the universe has the slightest interest in intelligence—or even in life. Both may be random accidental by-products of its operations like the beautiful patterns on a butterfly's wings. The insect would fly just as well without them."
—The Lost Worlds of 2001, 1972