"John Locke explains that the idea of substance is how we know things exist outside of our minds. He believed that substance is 'what can exist on it’s own,' not dependent on another”(Philosophical Conversations, Melchert, Chapter 10, page 281). He believed this was a crucial piece to proving that things we experienced actually existed outside of our minds. He spoke to the power of substances, using the example of a magnet attracting iron fillings. If we know that something has a power to change another thing, we know it has substance and exists."
Discerning exactly what that substance is, however, is an entirely different quality. If sensate qualities are all that we can perceive (without possessing in any way entrance into, or observation of, the substance) then we are left with the problem that Hume had concerning qualities and then what stands behind them. Hegel addresses Hume's problem in The Phenomenology of Spirit (the sugar cube example).
It is for this reason that I believe German Idealism was "already beyond this problem" and indeed has already moved beyond any correlationist-appearance nonsense. A close study of Fichte, or Hegel for that matter, clearly proves this beyond a shadow of a doubt.