A firearm is the only personal weapon that puts a 100-pound woman on equal footing with a 220-pound mugger, a 75-year old retiree on equal footing with a 19-year old gang banger, and a single guy on equal footing with a carload of drunk guys with baseball bats. The gun removes the disparity in physical strength, size, or numbers between a potential attacker and a defender.
When I carry a gun, I don't do so because I am looking for a fight, but because I'm looking to be left alone. The gun at my side means that I cannot be forced, only persuaded. I don't carry it because I'm afraid, but because it enables me to be unafraid. It doesn't limit the actions of those who would interact with me through reason, only the actions of those who would do so by force. It removes force from the equation... And that's why carrying a gun is a civilized act.