In “The Voices Within,” Charles Fernyhough delves into the concept of “inner speech,” more commonly known as talking to oneself. Fernyhough asserts that inner speech develops alongside social speech, following a much faster and more condensed format than verbal communication.
  

Key Points: 

1. Charles Fernyhough, a professor at Durham University in the UK, provides a historical analysis of “inner speech” in his book, “The Voices Within.”
2. The concept of inner speech, or self-talk, was first studied by Lev Vygotsky, a Russian psychologist, in the 1920s, who noted that as children learn to converse with others, they also develop the ability to converse with themselves.
3. Inner speech is not restricted by the typical conventions of verbal communication and can occur much faster, estimated at an average pace of 4,000 words per minute, which is 10 times faster than verbal speech.
4. Inner speech often takes a more condensed form than verbal speech as we do not need to use full sentences when conversing with ourselves, owing to our innate understanding of what we mean.

Source: https://www.theatlantic.com/science/archive/2016/11/figuring-out-how-and-why-we-talk-to-ourselves/508487/