Ghost: something is over and done with; a past that can't be changed; the invisible enemy or something that you can't get a grasp or see; something more spiritual than physical.
YOU CAN NEVER FIGHT A GHOST means:
1.If something is over and done with, you can't keep on fighting it, because it is over. If someone has died, it would be futile for their loved ones to try to keep them alive (which is obviously impossible).
2. You can't fight or overcome an invisible enemy or something that you can't get a grasp on (because ghosts are transparent).
3.You can never fight something that you can't see. Marry a Widow...you'll find that her late husband was a saint...no matter what she said while he was alive.
4.You can't change the past.
5.You cannot fight against someone or something that is more spiritual than physical...for example, you can't defeat a martyr
6. You can't change the past, what mistakes were made, etc just learn from them and try not to make the same ones again