In short, emphasizes Nietzsche, "la11gue1ge is rhetoric, because it desires to convey only a doxa [opinion], not an episteme [knowledge]."
With the marginal note from Nathaniel in mind, this binary is really interesting (and necessary) to unpack. I've had to read a lot of Foucault lately, so I'm thinking with him through a lot of my other readings right now. But his use of episteme, in some ways, breaks down that binary. By treating an episteme as the "epistemological unconscious" of an era (meaning that some knowledge and some assumptions are so inherent at a specific time and place that society doesn't even know it's happening), Foucault seems to suggest that opinion and knowledge can uniquely shift and intertwine in each epoch (again, within a culture that doesn't even know it's happening).