Imagine you have a radio that no matter how you turn the knob picks up only one station. You have no control over the volume. At times the sound is barely audible, at others, it is so loud that it almost shatters your eardrums. Moreover it is impossible to turn it off; at times it will be slow; it will suddenly begin to blare away when you want to rest and sleep. Who would put up with this kind of performance in a radio? And yet when your heart behaves in this kind of crazy fashion you not only put up with it but even call it normal and human.
