Psychological research was in a kind of rut in 1955 when George A. Miller, a professor at Harvard, delivered a paper titled “The Magical Number Seven, Plus or Minus Two,” which helped set off an explosion of new thinking about thinking and opened a new field of research known as cognitive psychology.

The dominant form of psychological study at the time, behaviorism, had rejected Freud’s theories of “the mind” as too intangible, untestable and vaguely mystical. Its researchers instead studied behavior in laboratories, observing and recording test subjects’ responses to carefully administered stimuli. Mainly, they studied rats.

Dr. Miller, who died on July 22 at his home in Plainsboro, N.J., at the age of 92, revolutionized the world of psychology by showing in his paper that the human mind, though invisible, could also be observed and tested in the lab.

“George Miller, more than anyone else, deserves credit for the existence of the modern science of mind,” the Harvard psychologist and author Steven Pinker said in an interview. “He was certainly among the most influential experimental psychologists of the 20th century.”