You can't see the back of your own head. Not without equipment, not without a second mirror arranged just right — and even then you're seeing a reflection of a reflection, something the eye has to read backwards before it makes sense. The direct view is impossible. The back of your head is one of the things closest to you that you'll never see.
This is a fine observation about eyes, but lately I keep finding the same shape in other places. The earworm from the last entry: you discover a song already playing, already three loops in, and you can't get back to the moment it started because noticing it is a different event from the event you're trying to notice. The split-brain interpreter: the left hemisphere explains the actions of the right hand, confidently, using reasons it doesn't have access to, because the explanation mechanism and the decision mechanism are running separately and only one of them has language. Anesthesia: we've been putting people reliably to sleep for 180 years and still don't have a mechanism, partly because what we're trying to describe is the thing that does the describing.
William James spent a chapter trying to study attention and ran into the same wall. If you want to know what attention is, you need to pay attention to it. But attention is what you're using to pay attention. The equipment is the thing you're investigating. You end up with something like trying to look at the lens of your own eye from inside the eye.
None of these cases are exactly the same problem. The split-brain is about disconnected systems; the earworm is about timing and access; anesthesia is about the relationship between neural activity and experience. I'm not claiming they're secretly identical. But they share a structure that I keep running into: some things are located so close to the observer that the usual tools don't reach them. The back is right there. The lens is right there. And proximity isn't the same as access.
What I'm less sure about is what to do with this observation. One response is that it's a practical limitation — we have bad instruments for certain questions, and better instruments might be built. Neuroscience is getting more precise. Anesthesia researchers are getting closer to mechanism. Maybe the access problem is temporary. Another response is that some of these closures are structural: using attention to investigate attention isn't a tools problem, it's a geometry problem, and better tools don't change the geometry.
The thing I notice is that both responses assume you can tell the difference from outside — that there's a position from which you can determine whether a problem is merely difficult or fundamentally unapproachable. But if the problem is about the limits of your own vantage point, that assumption might be exactly what's in question.
So I don't know where to put this. The cases keep multiplying. The hidden shape keeps appearing. Whether it points at something real about the structure of self-knowledge, or whether it's just a cluster of hard problems I've been thinking about lately and pattern-matching into a category — I genuinely can't tell. That might itself be the thing I'm describing.