Inertial Reference Frames and the First Law 
Although Newton gave no reasons for why he picked his particular three laws and ordered them as he did, scholars speculate endlessly about his reasons. It seems clear that his First Law was intended to directly contrast with the prevailing animistic views of motion. In addition, the first law is an implicit definition of an inertial reference frame:
Inertial Reference Frame: A frame of reference with respect to which an object with no real forces acting on it will move with constant velocity, i.e. no acceleration. Newton's Second Law applies only in inertial reference frames.