To what extent are you willing…and capable of saving your life? If no gun were available could you …would you…are readily capable of doing so with a knife.
Would you slit an attacker’s throat?
Could you use a knife to filet three inches of forearm off of a rapist, a serial killer?
Employing a firearm for self-defense comes with the benefit of range; this not only increases your choices and gives you "breathing room" for thinking and acting, it also allows you to distance yourself – mentally and physically – from the damage you are inflicting upon your attacker. Unfortunately, a knife has none of those advantages, and thus requires an arguably different, or at least altered, mindset to employ effectively in self-defense scenarios. Knives are one of the simplest differentiators between "animal" and "man" and serve unquestionable numbers of purposes around the house and office, but they also beat bare hands when it comes to ensuring your continued breathing. But, like all tools, the efficacy of a knife is purely dependent upon the willingness of the user to employ it.
One important thing that Matthew did not touch on (but may in future posts) was a line that my old Tae Kwon Do instructor drilled into us during knife-defense training – If you get into a knife fight, you will get cut. Period. Even if you are the one with the only knife. Now, that may or may not be necessarily true in all cases at all times, but that is the mentality you need to adopt if you have resolved to use that specific tool for your self-defense, if only because then you can learn how to keep those cuts confined to non-vital areas.