This paper is part of a chapter from a book I am writing on the concept of self-opacity, when we do not know or understand our own minds. In this paper, I develop a suggestive, puzzling comment from Stanley Cavell, who writes that “[the mind] receives its proof of its existence in the only form in which that psyche can believe it, namely as essentially unknown to itself.” What does it mean for the mind to receive a “proof” of its existence? And why would its being unknown to itself constitute such a proof? I argue that encounters with the mind’s opacity are ways of apprehending the mind’s robust reality, of realizing that my mind has a reality that outstrips my consciousness of it. I look at two cases: when you stumble upon (Barbara Herman) some aspect of your own mind that had been opaque; and when somebody else tells you something you about yourself that had been opaque. I suggest that while such encounters with the mind’s opacity can be disorienting, they can be “reality-conferring.” I close the paper with a discussion of the Nietzschean proposal that the mind’s opacity or unknownness to itself might be approached not just as an empirical-psychological fact about us, but as something to value or affirm.