I was reading a physics textbook. It is stated that force is defined by mass and acceleration (a force of 1N causes a mass of 1kg to accelerate at 1ms^-2). In later part, it is stated that mass is defined by force and acceleration (a mass acted by a force of 1N accelerates at 1ms^-2 is 1kg). As force is defined by mass, mass has to be defined first. But mass is defined by force. So it's sth circular and I am confused. Also, Newton's 2nd law (and hence Newton's 1st law too) is only true for inertial frames. However, whether a frame is inertial is determined by whether Newton's 2nd law is true for that frame. Again it's sth circular and I'm am confused. Can anyone please clear my concept?