The rationale of the separation of powers is often elided with the rationale of checks and balances and with the rationale of the dispersal of power generally in a constitutional system. This paper however focuses resolutely on the functional sepaartion of powers in what MJC Vile called its "pure form". Rexeamining the theories of Locke, Montesquieu, and Madison, the paper seeks to recover (amidst all their tautologies and evasions) a genuine case in favor of this principle. The paper argues that the rationale of the separation of powers is closely related to that of the rule of law: it is partly a matter of the distinct integrity of each of the separated institutions (courts, legislature, and administration). But above all, it is a matter of articulated governance (as contrasted with compressed undifferentiated exercises of power).