Re: Mutazilites
In my humble opinion, both Rasool and Nabi had the same connotation and understanding at the time of Holy Prophet and were in use both simultaneously. The main difference was in fact from their derivation. The word Rasool was/is derived from the word, Arsal which means "something that has been sent" and/or Arsaal which means "to send". Thus Rasool means "The one who has been sent" and "Rasool Allah means the one sent by Allah Almighty". On the other hand the word Nabi was/is derived from the word Naba' which means any message, news, happening etc not known to others. It is in contrast to Khabar which is any message, news, happening etc known by some who have witnessed it or known it from others. So the word Nabi means the one who tells something which others know not. So Holy Prophet Muhammad (MPBUH) was a personality, "Who had been sent by Allah Almighty" to "Tell the truth about Allah, Angels, Hell, Heaven, Judegment Day etc, not known to most of the people of that time".
I have never read an explanation like this, it really appeals to me sense of intellect. Can you please go ahead and further explain the difference between a messenger and a prophet based on the same lines.