All the talk about "don't carry a knife if you're not trained" is bullshit, completely ignore it. Why? Here me out:
1. Pocket knives are a utility tool and have more uses than just self-defense
2. The chances you are put in a situation against another person/people who have been trained and are up-to-date on hand-to-hand combat and knife fighting is slim for the average person. I'm not talking about operators operating at zero-dark thirty, but the average person going about their day. I walk my dog every afternoon/evening. I carry a pocket knife every time.
3. Wild animals have an evolutionary advantage over us, but our tech/brain is the great equalizer, use it. Bears have claws and teeth. Dogs/coyotes/wolves have teeth and strong jaws, etc. We have tools. Use them. A blade will do significantly more damage against an opponent than your hands/feet, unless you've been trained and are fit and happen to be gifted with physique/strength/size. Even a 5'3" woman with a blade can be dangerous to a 6'2" man with an 80-100lb weight difference.
4. The excuse "it could be used against me" is another BS excuse. If you're in a situation where you need a knife for protection, not having one is infinitely worse than having one and could be a life saver. It's no different than carrying a firearm. Either/both could be turned against you.
5. Practice and get some training if you don't have any. While I argue above that you should still carry a knife regardless, we preach training for firearms. It's the same for knives if you plan on using one for self defense. At a minimum, learn some basics so you don't get disarmed/knocked on your ass immediately.
6. Keep a knife as part of a multi-layered strategy, not the only strategy. I.e. escalation of force/different situations: mace, stun gun, knife, gun for instance. I'm not saying you'll have time to use all four, I'm saying being prepared for any/all contingencies is probably the safest way to prepare.