Clive Staples Lewis commonly used the nickname Jack.
Lewis did not generally use “Clive” in everyday life. According to accounts of his childhood, a childhood dog named Jacksie was killed, and the young Lewis insisted on being called Jacksie. The name was eventually shortened to Jacks and then Jack, which became his familiar personal name.
Friends, family members, and colleagues therefore usually knew him as Jack Lewis rather than Clive Lewis. “C.S. Lewis” remained the professional form attached to his books, lectures, and academic work. The nickname is especially useful for distinguishing private references to the man from formal references to the author.
Warren Lewis, Jack’s older brother, was another important figure in his life. The brothers shared literary interests and later lived together for many years. “Jack” is not a pen name for a separate writer; it was Lewis’s preferred personal nickname.