In 1912, Külpe published Über die moderne Psychologie des Denkens, which translates into English as On the Modern Psychology of Thinking. In this book, he was looking back upon his focus on the systematic experimental introspection method. He believed that before systematic experimental introspection existed, research on thought was incomplete. He also believed that having the subject report only on sensations, feelings, and presentations in thought research was severely limiting in that it did not provide an acceptable opportunity to identify what could be considered neither sensation, feeling, nor presentation. Once they were given the opportunity to objectively self-observe and describe what was neither sensation, feeling, nor presentation, yet was still a thought process; Külpe and his colleagues identified the need for new definitions and concepts aside from those that already exist. As Külpe wrote: