Full Idea
The natural logic of capitalism leads to democracy. For economic liberties without political liberties are inherently unstable.
Gist of Idea
Economic capitalist liberty naturally leads to democratic political liberty
Source
Michael Novak (The Spirit of Democratic Capitalism [1982], p.15), quoted by David Conway - Capitalism and Community II
Book Reference
'The Communitarian Challenge to Liberalism', ed/tr. Paul,E., Miller,F. /Paul,J. [CUP 1996], p.140
A Reaction
Not sure about 'inherently unstable'. Only in the sense that if you dish out economic liberties, people come to expect political liberties. China and Hong Kong are each currently facing this problem. Capitalist liberty can be highly restricted.