“I do not design a machine which will give the ignorant in astronomy a just view of the solar system, but would rather astonish the skilful and curious observer by a most accurate correspondence between the situations and motions of our little representatives of our heavenly bodies and the situations and motions of those bodies themselves. I would have my orrery really useful by making it capable of informing us truly of the astronomical phenomena for any particular point of time, which I do not find that any orrery yet made can do.”

Such a statement is indicated as his response to a question regarding the financial fiasco known as the South Sea Bubble; the earliest mention of this famous anecdote appears to be from manuscripts of the Second Memorandum Book (1756) of Joseph Spence, first published in Anecdotes, Observations, and Characters, of Books and Men (1820) https://archive.org/details/anecdotesobserv00singgoog edited by in Samuel Weller Singer; a Lord Radnor is quoted as saying:
When Sir Isaac Newton was asked about the continuance of the rising of South Sea stock? — He answered, "that he could not calculate the madness of the people."
Variants:
I can calculate the motions of erratic bodies, but not the madness of a multitude.
As quoted in "Mammon and the Money Market", in The Church of England Quarterly Review (1850), p. 142 http://books.google.com/books?id=s_cDAAAAQAAJ&pg=PA142&dq=%22but+not+the+madness%22&hl=en&ei=nUtbTfuoCYG6ugPFi4n4DA&sa=X&oi=book_result&ct=book-preview-link&resnum=1&ved=0CCkQuwUwAA#v=onepage&q=%22but%20not%20the%20madness%22&f=false
I can calculate the motions of the heavenly bodies, but not the madness of people.
I can calculate the motions of heavenly bodies but not the madness of men.
I can calculate the movement of the stars, but not the madness of men.
Disputed

Wie steht es bei dem Kreisen der sogenannten Elektronen um ihren zentralen Kern? Was ist hier wirklich unmittelbar wahrgenommen worden? Nichts von den bewegten Teilchen; was vielmehr beobachtet wurde, sind Erscheinungen, welche auf den ersten Blick mit der Bewegung von Körpern gar nichts zu tun haben. Alles übrige, was zum Atommodell geführt, ist eine lange Kette von Schlüssen.
In an address to the Viennese Chemisch-Physikalische Gesellschaft http://www.cpg.univie.ac.at/, April 26, 1932, as quoted by [Joseph Braunbeck, Der andere Physiker: das Leben von Felix Ehrenhaft, Leykam Buchverlagsgesellschaft, 2003, 3701174709, 51]

Michael Knipe, "Mr Smith agrees to majority rule coming within two years", The Times, September 25, 1976, p. 1.
Statement (September 24, 1976) on negotiations in South Africa which proposed a phased transition to majority rule.

On geomatric motion. A History of the Work Concept: From Physics to Economics, by Agamenon Oliveira, p. 154.

Source: Sir William Herschel: His Life and Works (1880), Ch.4 "Life and Works" quote from his paper "Nature and Construction of the Sun and Fixed Stars" (1795).
Context: I should not wonder if, considering all this, we were induced to think that nothing remained to be added; and yet we are still very ignorant in regard to the internal construction of the sun.... The spots have been supposed to be solid bodies, the smoke of volcanoes, the scum floating on an ocean of fluid matter, clouds, opaque masses, and to be many other things.... The sun itself has been called a globe of fire, though, perhaps, metaphorically.... It is time now to profit by the observations we are in possession of. I have availed myself of the labors of preceding astronomers, but have been induced thereto by my own actual observation of the solar phenomena.<!-- p. 145-146

Source: Sir William Herschel: His Life and Works (1880), Ch.4 "Life and Works" quote from "Researches on the Motion of the Sun and of the Solar System in Space" (1782), p. 149

Ackoff (1999, p. 34) cited in: Michael C. Jackson (2000) Systems Approaches to Management. p. 234.
1990s

Address to the Society for Psychical Research (1897)
Context: I will point out a curious, inveterate, and widespread illusion — the illusion that our earthly bodies are a kind of norm of humanity, so that ethereal bodies, if such there be, must correspond to them in shape and size.
When we take a physical view of a human being in his highest form of development, he is seen to consist essentially of a thinking brain, the brain itself, among its manifold functions, being a transformer whereby intelligent will power is enabled to react on matter. To communicate with the external world, the brain requires organs by which it can be transported from place to place, and other organs by means of which energy is supplied to replace that expended in the exercise of its own special functions.

Book II: Astronomy, Ch. I: General View
The Positive Philosophy of Auguste Comte (1853)