Daniel Keyes, the author of the classic novel Flowers for Algernon, passed away at the age of 86 on Sunday.

His daughter Leslie Keyes said that her father died as a result of complications from pneumonia, The Associated Press reports.

Even at an early age, Keyes wanted to be a writer, but was pushed into enrolling at New York University's premedical program by his parents.

In 1959, Keyes published Flowers for Algernon first as a short story, which is written through the eyes of Charlie Gordon, who has a low IQ, as journal entries. He enrolls in an experiment where his intelligence is improved as the doctors look to perform the same surgery they did on Algernon, a lab mouse.

Just like the mouse, Charlie becomes more intelligent, which is reflected through the book by smarter, less error-filled writing, before the mouse regresses and then dies, reflecting the protagonist's future.

Keyes turned the Hugo Award-winning short story into a Nebula Award-winning novel, which has sold over 5 million copies and was adapted into the 1968 film Charlie, which won Cliff Robertson the Oscar for best actor.

According to The New York Times, the idea for the story came as he was going through premed.

"I thought: My education is driving a wedge between me and the people I love," Keyes wrote in his autobiography Algernon, Charlie and I. "And then I wondered: What would happen if it were possible to increase a person's intelligence."

The character of Charlie then was created by Keyes as he was instructing a special needs class. In a 1999 interview, the author explained a student came up to him and expressed the desire to be moved out of the "dummy's class."