“I cannot know.”
Agnosticism is a valid, default response to aspects of a set of well-defined problems: That which is known is known; that which is unknown is also known. The agnostic position, then, recognizes where a particular question is not adequate. It sees that the question is the problem. One should change the question; look for a way to formulate a better question.
When you are not aware of what you don’t know, not even aware that you don’t know, can it ever be true that an assertion like “I cannot know” could be valid or meaningful? Further, how about that murky, that which you do not know that you know: What you knew, but forgot; knew, but merely took as granted; knew, but without context or analysis? Your unthought presuppositions and prejudices.
It is this gap in knowledge, I think, that is the real state of permanent, tragic bafflement. This gap leaves the “cannot know” mindset with no tools and no approach in what could appear to be an infinitely complicated and permanently indecipherable world.
Actually, those problematic states of ignorance — ignorance about “what you do not know” and ignorance about “what you do know” — are dangerous conditions to find yourself in. The latter is the worse of the two, I think. Both propel tepid, pseudo-intellectual agnosticism on a plummeting toward self-imposed ignorance!