Article 2, Section 1, Clause 1
Document 17
John Adams to Timothy Pickering
31 Oct. 1797Works 8:560The worst evil that can happen in any government is a divided executive; and, as a plural executive must, from the nature of men, be forever divided, this is a demonstration that a plural executive is a great evil, and incompatible with liberty. That emulation in the human heart, which produces rivalries of men, cities, and nations, which produces almost all the good in human life, produces, also, almost all the evil. This is my philosophy of government. The great art lies in managing this emulation. It is the only defence against its own excesses. The emulation of the legislative and executive powers should be made to control each other. The emulation between the rich and the poor among the people, should be made to check itself by balancing the two houses in the legislature, which represent these two classes of society, so invidious at all times against each other.
The Founders' Constitution
Volume 3, Article 2, Section 1, Clause 1, Document 17
http://press-pubs.uchicago.edu/founders/documents/a2_1_1s17.html
The University of Chicago Press
The Works of John Adams. Edited by Charles Francis Adams. 10 vols. Boston: Little, Brown & Co., 1850--56. See also: Butterfield; Cappon; Warren-Adams Letters