“The accumulation of all powers, Legislative, Executive, and Judiciary, in the same hands, whether of one, a few, or many, and whether hereditary, self-appointed, or elective, may justly be pronounced the very definition of tyranny.”
Federalist No. 47 (30 January 1788) Federalist (Dawson)/46 Full text at Wikisource http://en.wikisource.org/wiki/The
Source: 1780s, Federalist Papers (1787–1788)
Context: One of the principal objections inculcated by the more respectable adversaries to the Constitution is its supposed violation of the political maxim, that the Legislative, Executive, and Judiciary departments ought to be separate and distinct. In the structure of the Fœderal Government, no regard, it is said, seems to have been paid to this essential precaution in favor of liberty. The several departments of power are distributed and blended in such a manner, as at once to destroy all symmetry and beauty of form, and to expose some of the essential parts of the edifice to the danger of being crushed by the disproportionate weight of other parts.
No political truth is certainly of greater intrinsic value, or is stamped with the authority of more enlightened patrons of liberty, than that on which the objection is founded. The accumulation of all powers, Legislative, Executive, and Judiciary, in the same hands, whether of one, a few, or many, and whether hereditary, self-appointed, or elective, may justly be pronounced the very definition of tyranny.
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James Madison 145
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Source: Science is Not Enough (1967), p. 28 - 29
Context: We puzzle as to whether the universe is bounded or extends forever; whether, indeed, it may only be one universe among many. We speculate as to whether our universe began in a vast explosion, whether it pulsates between utter compression and wide diffusion, whether it is self-renewing and thus unchanged forever. And we are humble.
But science teaches more than this. It continually reminds us that we are still ignorant and there is much to learn. Time and space are interconnected in strange ways; there is no absolute simultaneity. Within the atom occur phenomena concerning which visualization is futile, to which common sense, the guidance from our everyday experience, has no application, which yield to studies by equations that have no meaning except that they work. Mass and energy transform one into another, Gravitation, the solid rock on which Newton built, may be merely a property of the geometry of the cosmos. Life, as its details unfold before us, becomes ever more intricate, emphasizing more and more our wonder that its marvelous functioning could have been produced by chance and time. The human mind, merely in its chemical and physical aspects, takes on new inspiring attributes.
And what is the conclusion? He who follows science blindly, and who follows it alone, comes to a barrier beyond which he cannot see. He who would tell us with the authority of scholarship a complete story of why we exist, of our mission here, has a duty to speak convincingly in a world where men increasingly think for themselves. Exhortation needs to be revised, not to weaken its power, but to increase it, for men who are no longer in the third century. As this occurs, and on the essential and central core of faith, science will of necessity be silent.
But its silence will be the silence of humility, not the silence of disdain. A belief may be larger than a fact. A faith that is overdefined is the very faith most likely to prove inadequate to the great moments of life.

Madison's notes (1 June 1787) http://avalon.law.yale.edu/18th_century/debates_601.asp
1780s, The Debates in the Federal Convention (1787)
Context: Mr. MADISON thought it would be proper, before a choice shd. be made between a unity and a plurality in the Executive, to fix the extent of the Executive authority; that as certain powers were in their nature Executive, and must be given to that departmt. whether administered by one or more persons, a definition of their extent would assist the judgment in determining how far they might be safely entrusted to a single officer. He accordingly moved that so much of the clause before the Committee as related to the powers of the Executive shd. be struck out & that after the words "that a national Executive ought to be instituted" there be inserted the words following viz. "with power to carry into effect the national laws, to appoint to offices in cases not otherwise provided for, and to execute such other powers "not Legislative nor Judiciary in their nature," as may from time to time be delegated by the national Legislature." The words "not legislative nor judiciary in their nature" were added to the proposed amendment in consequence of a suggestion by Genl. Pinkney that improper powers might otherwise be delegated.

“The combination of economic and political power in the same hands is a sure recipe for tyranny.”
“Introduction”, p. 3
Free to Choose (1980)