Although, the Constitution does not expressly mention the “separation of powers,” the government created by the 1787 U.S. Constitution presupposes and gives expression to it. Below are quotes from the founding generation on why the separation of powers is referred to by Madison in The Federalist No. 47 as an “invaluable precept in the science of politics.”

(1) “The chief improvement in government, in modern times, has been the compleat separation of the great distinctions of power; placing the legislative in different hands from those which hold theexecutive; and again severing the judicial part from the ordinaryadministrative. ‘When the legislative and executive powers (says Montesquieu) are united in the same person, or in the same body of magistrates, there can be no liberty.'” – Centinel No. II (October 24, 1787)

(2) “Liberty therefore can only subsist, where the powers of government are properly divided, and where the different jurisdictions are inviolably kept distinct and separate. . . . This doctrine is not novel in America, it seems on the contrary to be every where well understood and admitted beyond controversy; in the bills of rights or constitutions of New-Hampshire, Massachusetts, Maryland, Virginia, North-Carolina and Georgia, it is expressly declared. “That the legislative, executive and judicial departments, shall be forever separate and distinct from each other.” InPennsylvania and Delaware, they are effectually separated without any particular declaration of the principle.” – “William Penn” No. 2 (January 3, 1788)

(3) “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. Were the federal constitution therefore really chargeable with this accumulation of power or with a mixture of powers having a dangerous tendency to such an accumulation, no further arguments would be necessary to inspire a universal reprobation of the system. I persuade myself however, that it will be made apparent to everyone, that the charge cannot be supported, and that the maxim on which it relies, has been totally misconceived and misapplied. In order to form correct ideas on this important subject, it will be proper to investigate the sense, in which the preservation of liberty requires, that the three great departments of power should be separate and distinct.” – The Federalist No. 47 (James Madison)

(4) “The conclusion which I am warranted in drawing from these observations is, that a mere demarkation on parchment of the constitutional limits of the several departments, is not a sufficient guard against those encroachments which lead to a tyrannical concentration of all the powers of government in the same hands.” – The Federalist No. 48 (James Madison)

(5) “But the great security against a gradual concentration of the several powers in the same department, consists in giving to those who administer each department, the necessary constitutional means, and personal motives, to resist encroachments of the others. The provision for defence must in this, as in all other cases, be made commensurate to the danger of attack. Ambition must be made to counteract ambition. The interest of the man must be connected with the constitutional rights of the place. It may be a reflection on human nature, that such devices should be necessary to controul the abuses of government. But what is government itself but the greatest of all reflections on human nature? If men were angels, no government would be necessary. If angels were to govern men, neither external nor internal controuls on government would be necessary. In framing a government which is to be administered by men over men, the great difficulty lies in this: You must first enable the government to controul the governed; and in the next place, oblige it to controul itself. A dependence on the people is no doubt the primary controul on the government; but experience has taught mankind the necessity of auxiliary precautions. This policy of supplying by opposite and rival interests, the defect of better motives, might be traced through the whole system of human affairs, private as well as public. We see it particularly displayed in all the subordinate distributions of power; where the constant aim is to divide and arrange the several offices in such a manner as that each may be a check on the other; that the private interest of every individual, may be a centinel over the public rights. These inventions of prudence cannot be less requisite in the distribution of the supreme powers of the state. But it is not possible to give to each department an equal power of self defence. In republican government the legislative authority, necessarily, predominates. The remedy for this inconveniency is, to divide the legislature into different branches; and to render them by different modes of election, and different principles of action, as little connected with each other, as the nature of their common functions, and their common de pendence on the society, it will admit. It may even be necessary to guard against dangerous encroachments by still further precautions. As the weight of the legislative authority requires that it should be thus divided, the weakness of the executive may require, on the other hand, that it should be fortified. An absolute negative, on the legislature, appears at first view to be the natural defence with which the executive magistrate should be armed. But perhaps it would be neither altogether safe, nor alone sufficient. On ordinary occasions, it might not be exerted with the requisite firmness; and on extraordinary occasions, it might be perfidiously abused. May not this defect of an absolute negative be supplied, by some qualified connection between this weaker department, and the weaker branch of the stronger department, by which the latter may be led to support the constitutional rights of the former, without being too much detached from the rights of its own department? If the principles on which these observations are founded be just, as I persuade myself they are, and they be applied as a criterion, to the several state constitutions, and to the federal constitution, it will be found, that if the latter does not perfectly correspond with them, the former are infinitely less able to bear such a test.” – The Federalist No. 51 (James Madison)

(6) “The same rule, which teaches the propriety of a partition between the various branches of power, teaches us likewise that this partition ought to be so contrived as to render the one independent of the other. To what purpose separate the executive, or the judiciary, from the legislative, if both the executive and the judiciary are so constituted as to be at the absolute devotion of the legislative? Such a separation must be merely nominal and incapable of producing the ends for which it was established. It is one thing to be subordinate to the laws, and another to be dependent on the legislative body. The first comports with, the last violates, the fundamental principles of good government; and whatever may be the forms of the Constitution, unites all power in the same hands. The tendency of the legislative authority to absorb every other, has been fully displayed and illustrated by examples, in some preceding numbers. In governments purely republican, this tendency is almost irresistable. The representatives of the people, in a popular assembly, seem sometimes to fancy that they are the people themselves; and betray strong symptoms of impatience and disgust at the least sign of opposition from any other quarter; as if the exercise of its rights by either the executive or judiciary, were a breach of their privilege and an outrage to their dignity. They often appear disposed to exert an imperious controul over the other departments; and as they commonly have the people on their side, they always act with such momentum as to make it very difficult for the other members of the government to maintain the balance of the Constitution.” – The Federalist No. 71 (Alexander Hamilton)

(7) “It is important, likewise, that the habits of thinking in a free country should inspire caution in those entrusted with its administration, to confine themselves within their respective constitutional spheres, avoiding in the exercise of the powers of one department to encroach upon another. The spirit of encroachment tends to consolidate the powers of all the departments in one, and thus to create, whatever the form of government, a real despotism. A just estimate of that love of power, and proneness to abuse it, which predominates in the human heart, is sufficient to satisfy us of the truth of this position. The necessity of reciprocal checks in the exercise of political power, by dividing and distributing it into different depositaries, and constituting each the guardian of the public weal against invasions by the others, has been evinced by experiments ancient and modern; some of them in our country and under our own eyes. To preserve them must be as necessary as to institute them.” – George Washington, Farewell Address (1796)