"She said, sharply, “Elvex.”

The robot’s head turned toward her smoothly. “Yes, Dr. Calvin?”

“How do you know you have dreamed?”

“It is at night, when it is dark, Dr. Calvin,” said Elvex, “and there is suddenly light, although I can see no cause for the appearance of light. I see things that have no connection with what I conceive of as reality. I hear things. I react oddly. In searching my vocabulary for words to express what was happening, I came across the word ‘dream.’ Studying its meaning I finally came to the conclusion I was dreaming.”

-Isaac Asimov, “Robot Dreams”