I'm asking when can you whip out your knife.
First, I am not a lawyer, so what I say may or may not apply to your locale, but I believe this is
generally correct.
The conditions that must exist for when you can legally whip out a knife are no different for when you can draw a firearm.
These are the three conditions:
Opportunity, Ability and Motive.
1. Opportunity: The aggressor must have the
immediate opportunity to inflict serious bodily harm or death (somebody with a tire iron threatening you from behind a 12 foot, barbed wire fence doesn't have the
immediate opportunity to attack you. If a few minutes later the punk has found a way around said fence and approaches you, things start to change.)
2. Ability: The person(s) threatening you must have the ability to inflict serious bodily harm or death upon you. Mere words and threats DON'T cut it. The assailant must either possess a weapon of some sort (club, gun, knife, RPG, etc) OR disparity of force
you can't use a weapon against someone who doesn't have a weapon, basically.
Not necessarily true. Disparity of Force. Disparity of force means you are outnumbered, or it may mean that the aggressor(s) are much bigger, younger and/or faster than you. Remember, real life aint hollywood. Fist blows can be lethal.
3. Motive. If you stop by my house and I show you my gun collection, I possess (1) the
opportunity to do you harm (within the walls of my home, you are most assuredly in range of my Finnish Mosin
- which represents the (2)
ability to do harm.) What is missing is the motivation, or intent to do you harm or death.
Of course, the reasonable person standard applies.
All three of the above must be present to justify use of deadly force. And in most (if not all) jurisdictions, the mere act of drawing a DEADLY weapon is considered by law automatically to be use of LETHAL FORCE. Again, just the act of drawing. If you draw your weapon and only 2 of the 3 conditions above exist,you
can be charged and convicted of Assault with a Deadly Weapon.
Now, that doesn't mean you shouldn't take the threat seriously. If you are able to distance yourself from the situtation, that would be the wisest course of action, all the while keeping your guard up (Col Jeff Coopers "Condition Orange, IIRC).