To Understand is not to Know

A common error: to presume that, because one understands an idea, one has learned the idea. When we understand a thing, we do no more than realize that this idea can enter into the edifice of our thoughts without us having to knock down any walls, but that does not prove that the thought has therefore entered — nor that it will remain within thereafter. Many ideas are like water: they leave one’s mind as readily as they enter.