Full Idea
Were the judicial power joined to the legislative, the life and liberty of the citizens would be subject to arbitrary power. For the judge would then be the legislator.
Gist of Idea
The judiciary must be separate from the legislature, to avoid arbitrary power
Source
Baron de Montesquieu (The Spirit of the Laws (rev. 1757) [1748], 11.06)
Book Reference
Montesquieu,Baron de: 'Selected Political Writings', ed/tr. Richter,Melvin [Hackett 1990], p.182
A Reaction
This is the key 'separation of powers', which seems to be a mantra for nearly all theories of the state.