In case the title is not clear enough. I recently realized that the politicians use a lot of other people to write their words for them so there are not many "original" inspirational quotes nowadays. Who is the earliest known example of that?
Or do we know that any majorly popular inspirational quote from the history is actually thought and written by someone else?
I only really know about US presidents so I can speak about them a little. It is a myth that US presidents completely wrote their own speeches. Even George Washington had help from great minds like John Jay and Alexander Hamilton. Lincoln is considered one of the most, if not the most rhetorically gifted president, and even he had help. The position of a full time presidential speechwriter is a relatively recent invention. In the past, most speechwriters held other jobs (mostly policy advising positions) in the White House and closely advised the president on public policy matters.
"Insofar as we know, the first president to hire a full-time speechwriter in the White House was Warren G. Harding. The man's name was Judson P. Welliver" source.
When writing a presidential speech, there are many drafts that are looked over by usually several dozen people. The president works closely with his speechwriters to articulate a position that will be supported by the people. Some presidents who are not as rhetorically gifted will get a larger portion of their speech from their speechwriters, but all presidents are closely involved in the process of writing a speech.
Most "original" quotes by presidents have been written by a team of people, including the president himself. That being said, there have been great examples of speeches written completely by the president such as the Gettysburg Address. Also, Robert F. Kennedy's speech after MLK was shot is fantastic and was written completely by RFK.