Fully knowing someone is difficult. It may take months, years, or even decades. Some people are hard to read. Other people are extremely vulnerable and open. At the end of the day however, the amount of time and effort we put in dictates how much we know. That does not just stop at relationships either.
If a student puts an insane amount of effort into studying, then they will most likely get a good grade. If a young man is playing ping pong day after day for years, then he would be better than the average player. If someone wants to get great at chess, then they would play for months on end and could very well be the king of the local club. This even applies to how human beings train their bodies in sports or martial arts.
You cannot hit what you do not know. What a wildly simple, yet profound quote. If you started training in any martial art and immediately jump into a live engagement in your everyday life, there is no guarantee of a victory. If you take a deep look at the founding principles of Jeet Kune Do from Bruce Lee, you will see that knowing and consistency are key. JKD is not a gimmick or a fun way to fight whoever you want. Lee trained nearly his entire life to get to a place where he started to form this non-traditional life and fighting style.
Science shows that our muscles literally have a familiarity with movement. It becomes second nature to the point where we do not have to think. Our strikes or our grapples react before we can bark the command from our brain. It seems like an afterthought, but once you spend even a month training, your neurological adaptations set in. Imagine spending 9 months pushing your body to the limit and on the other side you get muscle memory in fights, a toned body, a disciplined mind, and a changed way of living. So with that, in life and in JKD, know what is coming at you and trust that the work you put in can get you through whatever obstacle.