“It is of the nature of our senses to be misled and deceived. Because they do not know what being is, they take ‘appears’ to be for ‘is.’” This appearance manifests in fleeting states; being is something permanent, unchanging. We are ever changing, always reformulating our souls based on actions and rationalizations of them.
This only presents us with a character to observe. Most of us never examine that character at all. But those that do often practice it poorly. This comes in the form of assumptions about the world, partly passed on through culture, partly by our own ignorance and lack of inquisitiveness. When we do the simplest acts, such as playing with a cat, we assume that we are manipulating the behavior of the cat, never considering the ways the cat is toying with us, or maybe just tolerating.
The admonition to ‘know thyself’ must take all of this into account, leading to the inevitable conclusion that we ultimately know nothing of importance, we merely observe this ever shifting nature and character. We can let this doubt drive us mad or into hopeless quests of certainly. But the better course is to accept the mystery for what it is and to stand in awe of our inability to master the world we inhabit.