Schopenhauer differed from Kant in that Kant argued the difference between "things as they actually are" and "things as they appear". Kant contended that we can never really know what an object or reality in general is like since our mind inevitably has to interpret these things which gives us no confidence that we actually perceive reality as it actually is. However, Schopenhauer accepted Kant's conclusion that the reality behind the world if appearances, or the noumenal world, is unknowable to the subjective self. However,
Schopenhauer went one step further, and in a way of speaking "ploughed around the stump or unknowability", arguing there is a back door into the world of things in themselves. Schopenhauer descried this alternative means of knowing reality as a way from within which stands open to us to the real inner nature of things to which we cannot penetrate from without. This is an interesting addition to Kant's conclusion. Kant looked outside the individual and concluded that we can never really know the true nature of reality. Schopenhauer looked within and argued that by doing so the individual has a means of grasping the actual or true nature of things.
This has been described some as Schopenhauer's subterranean passage through which we have access to actual reality. He argued that Kant had failed to see that which is implicit within his conclusion - that the Self or the "I" is actually part of that reality which we strive to know. Schopenhauer further concluded that this "I" is actually the "Will." In other words, the "Will" is the thing in itself, which is manifested in the will to live. This Will does not actually belong to the individual but is a presence within the person and as a presence within the individual it is striving to make itself known through the world of appearances.
