Lack of understanding [is] called stupidity; deficiency in the application of the faculty of reason to what is practical we [recognize] as foolishness; deficiency in power of judgement as silliness; finally, partial or even complete lack of memory as madness... That which is correctly known through the faculty of reason is truth. -From The World as Will and Representation, First Book, Section 6.
As moralizing as it sounds, Schopenhauer is not being so here. This is his way of defining the terms of discussion. As I continue reading, it is remarkable to notice how he uses the Principle of Sufficient Reason as a sort of philosophical bedrock to plant his philosophy and as a curious Occum's razor to untangle old epistemological knots. Einstein was greatly influenced by Schopenhauer's ideas of space, time and causality. Perhaps, one reason why he so stubbornly rejected the probabilistic interpretation of quantum mechanics.