Do you? You know that a gun could be a lethal weapon, but there are many, many factors you are personally unfamiliar with because you have no practical experience. From, whether or not you have it in you to pull the trigger and end someone's life, to whether you could maintain composure under duress. You know that guns can be lethal, but you do NOT know for a fact that you could do it.
Yes, I do know they work. I understand basic physics, basic anatomy, and my own capabilities such that I know they work.
I understand the logic you are trying to play out, but I disagree with its application to reality.
If we only know if something works after we have done it, then that means even if someone has shot a person, broken someones arm, put them to sleep in an RNC... they still cannot know if that will work next time, too many variables, maybe the next person will be wearing armor, maybe they'll be stronger, maybe the PTSD I experience from shooting and killing someone the last time will prevent me from pulling the trigger next time, maybe they'll know a defense against the RNC I wasn't expecting.... so by that logic we cannot know if our techniques are effective even if we've used them a dozen times before... so we'll never know anything.
By that logic, Monday morning I cannot know the same route I drive every day will get me to work, until I have actually arrived at work. After all there could be unknown hazards in the way.
By that logic, I cannot know that Interstate 90 will take me from Spokane to Seattle if I've never driven it, even though I know how driving a car works and I know how maps work, and other people have successfully done it many times...I've never done it, so I cannot know it works.
By that logic, I cannot know the light switch will work until I flip it, even though I've used it successfully 1,000 times previously.