system
Given the following post, respond as if you're commenting on Reddit:
human
Question about Hume, Kant, Hegel, Schopenhauer and Nietzsche

I've heard it said that to understand Nietzsche you need to read Schopenhauer. To understand Schopenhauer you need to read Hegel. To understand Hegel you need to read Kant. And to understand Kant you need to understand Hume. (Possibly back to Aristotle, but I'm not sure). 

Are all these references in regards to their metaphysics? What is being said with this idea?
gpt
Nietzsche takes Schopenhauer's notion of The Will and develops it, and it helps to know where he's coming from in that. 

I'd disagree that Hegel is necessary for Schopenhauer, because he wholesale rejects Hegel and all the other post-Kantians. But knowing Kant's system is essential and he even says as much in *The World as Will and Representation*. 

And to fully understand Kant it does really help to understand the context of the problems he's trying to solve. He's coming after a seeming impasse between European 'rationalist' thinkers and British 'empiricists' (Of whom Hume is the major figure), and Kant's system tries to overcome with deadlock. Kant himself says it was reading Hume which woke him from his 'dogmatic slumber' as he says.