“ For the intellect is, as has been shown, conditioned by nature, lies in it, belongs to it, and cannot therefore place itself over against it as something quite foreign to it, in order thus to take up into itself its whole nature, absolutely, objectively, and thoroughly. It can, if fortune favours it, understand all that is in nature, but not nature itself, at least not directly. ”
 Arthur Schopenhauer, The World as Will and Representation (1819). copy citation

add

“on which account the origin of the world does not lie within its sphere. It is accordingly limited to the relations between the things which lie before it, and is thus sufficient for the individual will, for the service of which alone it appeared. For the intellect is, as has been shown, conditioned by nature, lies in it, belongs to it, and cannot therefore place itself over against it as something quite foreign to it, in order thus to take up into itself its whole nature, absolutely, objectively, and thoroughly. It can, if fortune favours it, understand all that is in nature, but not nature itself, at least not directly. However discouraging to metaphysics this essential limitation of the intellect may be, which arises from its nature and origin, it has yet another side which is very consoling. It deprives the direct utterances of nature of their unconditional validity, in the assertion of which naturalism proper consists.” source