Full Idea
The double knowledge of the nature and action of our own body is the key to the inner being of every phenomenon in nature.
Gist of Idea
We know reality because we know our own bodies and actions
Source
Arthur Schopenhauer (The World as Will and Idea [1819], I 105), quoted by Peter B. Lewis - Schopenhauer 4
Book Reference
Lewis, Peter B.: 'Schopenhauer' [Reaktion Books 2012], p.93
A Reaction
Lewis calls this 'the heart of his philosophy'. Bodily awareness comes from acts of willing. So Lewis says 'the thing-in-itself is revealed to us in willing'. We experience Being and causation. Is he trying to combine idealism with the thing-in-itself?