I can calculate the movement of the stars, but not the madness of the people.

Isaac Newton (1642-1727) English physicist and mathematician
(Attributed)
    (Source)

Supposedly after the ruinous stock price collapse of the "South Sea Bubble" in 1720, in which Newton lost £20,000.

The earliest mention of this is found in Joseph Spence, Second Memorandum Book (1756), collected in Joseph Spence (ed. Samuel Weller Singer), Anecdotes, Observations, and Characters, of Books and Men (1820). There 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.'" (Note that this supposedly takes place before the bubble bursts.)

Variants:

• I can calculate the motions of erratic bodies, but not the madness of a multitude. ["Mammon and the Money Market," The Church of England Quarterly Review (1850)]
• 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 stars, but not the madness of men.