OrionLM

Hello! I am a 17 year old music student from Belgium. I recently wrote a concerto for double bass, and it turned out really well in my opinion. Now that it's finished I actually really want to write a new piece, but I can't find any themes. I was wondering, for those who don't really those "Aha! I've got a theme!"-moments at random, how do you find/look for a new theme? Where do you get your inspiration from? And how do you know if you didn't steal it from an other work? Thanks!