Although Aristotle is thought of as the first to speak of it, and John Locke and Montesquieu perfected it, the real original source of "separation of powers" appears in this weeks' Parsha: the priesthood (judicial), the king (executive) and the prophet (divine compass and conscience).
Though they aren't identical to the modern executive, judicial, and legislative, they do appear as 3 separate authorities and Halacha goes on to establish clear checks and balances between the three. 3 things I find very interesting are:
1. Once prophesy ended (second temple period), we still find 3 branches- priesthood (religious), the king (executive) and Sanhedrin (legislative and judicial).
2. In the Parsha there is no definition of the role of the king. All there is, is a list of prohibitions: not to have too much money, wives and horses, not to return to Egypt and not to forget the Torah. This is because the institution of the king is a secular institution in the sense that it deals with the mundane and secular parts of life - statehood, international relations, economy, etc... and therefore holds a higher potential of drifting from Torah.
3. Moshe Rabbeinu held all three branches but anyone else in Jewish history who tried to do so - failed (e.g. Uziya king of Judea, the Hasmonians, Aharon Barak...). (218)