Arthur Schopenhauer, an early 19th century philosopher, made significant contributions to metaphysics, ethics, and aesthetics. His work also informed theories of evolution and psychology, largely through his theory of the will to power – a concept which Nietzsche famously adopted and developed.
His brilliance, poetry, and crushing pessimism can be seen immediately in this work, as for example in this claim from the first chapter: "The pleasure in this world, it has been said, outweighs the pain; or, at any rate, there is an even balance between the two. If the reader wishes to see shortly whether this statement is true, let him compare the respective feelings of two animals, one of which is engaged in eating the other."