Open the pod bay doors, Hal.

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.

... chemistry is a trade for people without enough imagination to be physicists.

All human plans [are] subject to ruthless revision by Nature, or Fate, or whatever one preferred to call the powers behind the Universe.

In this single galaxy of ours there are eighty-seven thousand million suns. [...] In challenging it, you would be like ants attempting to label and classify all the grains of sand in all the deserts of the world. [...] It is a bitter thought, but you must face it. The planets you may one day possess. But the stars are not for man.

They had not yet attained the stupefying boredom of omnipotence; their experiments did not always succeed.

That's one of those meaningless and unanswerable questions the mind keeps returning to endlessly, like the tongue exploring a broken tooth.