Jovaro is right, Tuppence.
The title of this thread is referring to a self-sustaining process, but there is no reason to assume that it couldn't have started on its own.
To give an example:
Nuclear fusion in a star (like our sun) is produced by nuclear fusion. I could say 'It takes fusion to make fusion' and this would be correct, but we know and observe that the 'first fusion' occurs naturally. Now why wouldn't a 'first cell' have formed naturally as well?