Full Idea
The central sermon of existentialism is essentially a romantic one, namely, that there is in the world nothing to lean on.
Gist of Idea
Central to existentialism is the romantic idea that there is nothing to lean on
Source
Isaiah Berlin (The Roots of Romanticism [1965], Ch.6)
Book Reference
Berlin,Isaiah: 'The Roots of Romanticism' [Pimlico 2000], p.142
A Reaction
He tracks this back to Kant's view that our knowledge of the world arises out of our own minds. So what is there to lean on? Rational consistency? Natural human excellence? God? Pleasure? Anonymous duty? I like the second one.