When you observe an apple, the part facing you is visible, while the part facing away from you is not; when you observe an object, there are parts that can be observed and parts that cannot. "Knowing" and "not knowing", "understanding" and "not understanding", "being able to do" and "not being able to do" are like a person in the sunlight and their shadow, they can never be separated. We can never fully know or fully understand, and thus we can never fully do after having "fully known" or "fully understood". "Not being visible" does not mean "not existing", nor does it mean "not having an effect"; therefore, the future cannot be fully anticipated, and actions will always carry some risk.