Full Idea
The identity of the subject of willing with that of knowing by virtue whereof ...the word 'I' includes and indicates both, is the knot of the world, and hence inexplicable.
Gist of Idea
The knot of the world is the use of 'I' to refer to both willing and knowing
Source
Arthur Schopenhauer (Abstract of 'The Fourfold Root' [1813], p.211-2), quoted by Christopher Janaway - Schopenhauer 4 'Self'
A Reaction
I'm struggling to see this as a deep mystery. If we look objectively at animals and ask 'what is their brain for?' the answer seems obvious. This may be a case of everything looking mysterious after a philosopher has stared at it for a while.
Book Reference
Janaway,Christopher: 'Schopenhauer' [OUP 2002], p.51