There is no experiment to prove that you are in motion
Explanation:
A frame of reference which has constant velocity is known as an inertial frame of reference. Motion is relative. One can detect one's motion only when one observes change in position with respect to a fixed body.
Thus, if you are in a spaceship moving at a constant speed in a straight line and unable to look outside, you would not be able to prove that you are moving. Everything within the spaceship would have same speed. If you will throw any object within the spaceship, then the parameters measured by you would also not show that the spaceship is in motion.
There's multiple reasons a defense lawyer may admire Newtons 3rd Law but mainly it's because whenever for example someone is accused or did something there's a reason and also that person's going to have help.