The Will to Power · Friedrich Wilhelm Nietzsche
523.
There is no greater error than that of making psychical and physical phenomena the two faces, the two manifestations of the same substance. By this means nothing is explained: the concept _"substance"_ is utterly useless as a means of explanation. _Consciousness_ may be regarded as secondary, almost an indifferent and superfluous thing, probably destined to disappear and to be superseded by perfect automatism-- When we observe mental phenomena we may be likened to the deaf and dumb who divine the spoken word, which they do not hear, from the movements of the speaker's lips. From the appearance of the inner mind we draw conclusions concerning invisible and other phenomena, which we could ascertain if our powers of observation were adequate for the purpose. For this inner world we have no finer organs, and that is why a _complexity which is thousandfold_ reaches our consciousness as a simple entity, and we invent a process of causation in it, despite the fact that we can perceive no cause either of the movement or of the change--the sequence of thoughts and feelings is nothing more than their becoming visible to consciousness. That this sequence has anything to do with a chain of causes is not worthy of belief: consciousness never communicates an example of cause and effect to us.
Text from Project Gutenberg, public domain in the United States.