Complex sentence: Erikson's greatest innovation was to postulate not five stages of development, as Sigmund Freud had done with his psychosexual stages, but eight, and then later added a ninth stage in his book "The Life Cycle Completed".
Split into simpler sentences: Erikson's greatest innovation was to postulate not five stages of development, as Sigmund Freud had done with his psychosexual stages, but eight. He then later added a ninth stage in his book "The Life Cycle Completed".