However, this notion of applying "identical" forces to different objects brings us back to the fact that we have not really defined what a force is. We can sidestep this difficulty with the help of Newton's third law, which states that if one object exerts a force on a second object, it will experience an equal and opposite force. To be precise, suppose we have two objects of constant inertial masses m 1 and m 2. We isolate the two objects from all other physical influences, so that the only forces present are the force exerted on m 1 by m 2, which we denote F 12, and the force exerted on m 2 by m 1, which we denote F 21. Newton's second law states that