I remember a few years ago there was a story in the news (and heavily dicussed on this forum) about a guy who successfully fought off a moutain lion with a folding knife. The guy was attacked by a full-grown moutain lion, and while getting badly chewed up he pulled out his pocket knife, opened the knife, and stabbed the lion to death with it. The man survived.
I often think of this incident when the subject of self-defense with a knife, and what it takes to be successful at it, comes up. People often say "A knife is worthless for self-defense." or "You need extensive training to use a knife." or "You need a hard-core ruthless mindset to successfully use a knife.", ect, ect, ect.
The guy who killed the moutain lion, he had no training in fighting moutain lions with a knife, he wasn't former Spec-ops or some hard-core, badass tough guy with steel nerves and a fearless heart. And I'm sure the last thing he expected that day was that he would have to fight for his life, against a moutain lion, using only a folding knife.
He was just a regular guy who found himself in a desperate situation. Fighting for his life against a strong, ruthless, fearless, determined, and heavily armed predator, using only what he had. And yet he lived to tell the tale. Was he lucky? Sure he was. But I believe he survived because he had a strong will to live, and a knife. Sometimes, that's all it takes.
Some of my knives are tools, some are for self-defense.