Some criticisms of Schopenhaur:
1) His epistemology is self- referentially refuting. That is, like Kant, he is claiming to know with his mind that reality cannot be known with the mind. He steps outside of his private reality and makes objective knowledge claims about the noumenal world, the real world as it is, “out there”.
2) To claim that ultimate reality is not rational is also self- referentially refuting. The assertion that all reality is irrational is to deny the reality of the very mind asserting this. Why should one think an aimless nonrational Will gives rise to a rational mind as a byproduct? On what grounds should it be resisted? There has to be a transcendent immaterial good that acts outside the Will and outside of ourselves to resist this nonrational force. Morality by it’s very nature not only makes rational distinctions but must have some connection with the noumenal world. For example, we cannot seperate the wrongness of killing an innocent person from the actual person and their ontological status as a valuable being. This involves stepping outside of the phenomenal world and into the noumenal. The wrongness of killing an innocent person involves much more than having an irrational thought. Otherwise, killing an innocent person would be the equivalent of making a mathematical error.
3) He chided Kant for applying causality to the noumenal world, but he did the same thing implicitly. He believed that Cosmic Will was the cause of the phenomena we observe. But how could he know this unless the principle of causality was applied?
4) His pantheistic understanding of a single, omnipresent, universal Will, as opposed to many individual wills, is very unconvincing. If good and evil have the same ultimate source, why pursue the good? Rather, the good must be somehow greater to warrant it’s pursuit. It’s horribly counter intuitive to imagine a Hitler and a Mother Teresa as having the same ultimate source- acting out it’s Will with human beings.