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Recently I've seem here one answer telling that Newton's first law really assures the existence of inertial reference frames. But how is that? I really can't see it. As I know, Newton's first law says:
Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it.
Now, what I've seem done is to use this to define that whenever this law holds in some reference frame we call it inertial. But how this law states that such a frame does exist?