Perception is what you interpret through your senses (see, hear, touch, taste, smell) and is significantly affected by the programs you have been surrounded with throughout your life. Those programs are country, culture, family, friends, and work colleagues. Plus of course anything else you are exposed to, e.g. TV, movies, media and the news.
This is why a group of people that see the same event happening will all report seeing something slightly different afterward. It is also why eye witness statements in criminal cases are NOT the best evidence. Yes, some can be accurate but they can just as easily not be. Eyewitnesses will perceive and understand what they are seeing through their own filters.
It takes a strong mind (mind over matter) to regard events as they are happening and not bring all their past baggage with them to influence the situation.
Knowing yourself should not be taken lightly as some airy-fairy notion. Instead, by knowing ourselves we can know our minds, control our emotions, and not allow our past to color our perception of the present.
How do you practice mind over matter?