The second great structural principle of American Constitutional Law is
supplied by the doctrine of the Separation of Powers. The notion of
three distinct functions of government approximating what we today term
the legislative, the executive, and the judicial, is set forth in
Aristotle's Politics,[29] but it was the celebrated Montesquieu who, by
joining the idea to the notion of a "mixed constitution" of "checks and
balances", in Book XI of his Spirit of the Laws, brought Aristotle's
discovery to the service of the rising libertarianism of the eighteenth
century. It was Montesquieu's fundamental contention that "men entrusted
with power tend to abuse it". Hence it was desirable to divide the
powers of government, first, in order to keep to a minimum the powers
lodged in any single organ of government; secondly, in order to be able
to oppose organ to organ.

In the United States libertarian application of the principle was
originally not too much embarrassed by inherited institutions. In its
most dogmatic form the American conception of the Separation of Powers
may be summed up in the following propositions: (1) There are three
intrinsically distinct functions of government, the legislative, the
executive, and the judicial; (2) these distinct functions ought to be
exercised respectively by three separately manned departments of
government; which, (3) should be constitutionally equal and mutually
independent; and finally, (4) a corollary doctrine stated by Locke--the
legislature may not delegate its powers.[30]

Prior even to Franklin D. Roosevelt this entire colligation of ideas had
been impaired by three developments in national governmental practice:
first, the growth of Presidential initiative in legislation; secondly,
the delegation by Congress of legislative powers to the President;