In 1903, Mark Twain comforted Helen Keller, who had been accused of plagiarizing her story “The Frost King,” telling her in a letter, “All ideas are secondhand, consciously and unconsciously drawn from a million outside sources.” He took a harder line on his own intellectual property, however, campaigning so vigorously for stringent copyright laws that the American Bar Association later recognized him for his efforts.

Thomas Aquinas was so absorbed in solving a philosophical problem while dining with Louis IX around 1269 that he believed himself to be in his own office; when the solution came to him, he slapped the table and called to his secretary—who was not present—to get ready to write. The king, “amazed and edified that a man’s mind could be so enraptured by the spirit that none of the body’s senses could disturb it,” summoned a scribe to record the revelation.

The ancient physician Galen catalogued the anxious delusions of his melancholic patients, including those of a man who “believes he has been turned into a kind of snail” and “runs away from everyone he meets lest his shell get crushed,” and those of another who “is afraid that Atlas, who supports the world, will become tired and throw it away, and he and all of us will be crushed and pushed together.”

An ongoing international study of people who have survived severe cardiac arrest has led researchers to believe that the brain experiences a “hyper-alerted state” after clinical death. This means, they theorize, that consciousness could continue after the body stops showing signs of life; a person may be able to hear and perceive the pronouncement of their own death.