Full Idea
A critique of our capacity to know is nonsensical: how should the tool be able to criticise itself when it can, precisely, only use itself for the critique? It can't even define itself!
Gist of Idea
We can't use our own self to criticise our own capacity for knowledge!
Source
Friedrich Nietzsche (Writings from Late Notebooks [1887], 02[87])
Book Reference
Nietzsche,Friedrich: 'Writings from the Late Notebooks', ed/tr. Bittner,Rüdiger [CUP 2003], p.76
A Reaction
I am inclined to answer that it seems impossible, but it happens. Thinking about ourselves is the hardest part of philosophy, but phenomenologists and others (starting with Descartes) have had an impressive crack at it. Nietzsche was good at it.