Natural science is a branch of knowledge by general consent not primarily based on the a priori. It […] observes and endeavours by observation to follow and trace the 'how' of what happens in Nature. It proceeds further to generalize about this 'how'. It tries to decipher something of it in the past and to forecast something of it in the future. Above all it expends its utmost pains on attempting to describe the 'how' fully and accurately by first-hand observation at this present.

The brain is a mystery—it has been—and still will be. Not that we do not know many facts about it. The facts we know have indeed greatly multiplied in recent years, but they all fail to give us a key to the mystery of how it creates—if it does create—our thoughts and feelings; that is, said more concisely though less concretely, our mind.