Personally, I feel that a knife is a terrible device for self defense or attack. Unless you have a 12" kabar sheathed to your belt, you probably won't be able to whip it out quickly enough to defend yourself, and I'm accounting for illegal things like switchblades. I was once attacked by a crazed guy with a knife, and I was able to disarm him quickly and break his arm for the inconvenience. Then again, 17.5 years of Isshin Ryu under your belt will give you that advantage. No matter what item(s) you use to defend yourself, if any, the point is that you don't have time to think about minimizing damage when someone is assailing you. If you fear for your safety, you take out the attacker without hesitating, because hesitation only gets victims killed.