I was reading "Reinventing Gravity - A Physicist Goes Beyond Einstein" by Mr. John Moffat, btw it's a really good book for understanding introductary level of classical and theoretical physics, anyway a passage there caught my eye. I'm not finished with book yet but the author expressed/chose his words carefully as he said "the questions related to source of inertia have yet to be answered." eventhough physicists like Einstein or others tried to express the situation but couldn't come up with an idea which could be acknowledged by scientists. Might sound like stupid but I couldn't find an answer on Wikipedia either. Is there a scientific consensus, sort of update, on this subject? I always thought inertia and gravity goes hand to hand as far as Newton's laws are concerned. Answers are appreciated. EDIT: Oops I might have opened this on a wrong section when I was reading through the threads, sorry about that.