“I have read somewhere or other, — in Dionysius of Halicarnassus, I think, — that history is philosophy teaching by examples.”
On the Study and Use of History, letter 2; in fact this relates to a third-century CE treatise on rhetoric, wrongly attributed to Dionysius of Halicarnassus, which says (xi. 2): "The contact with manners then is education; and this Thucydides appears to assert when he says history is philosophy learned from examples". The line is not found in Thucydides.
Help us to complete the source, original and additional information
Henry St John, 1st Viscount Bolingbroke 10
English politician and Viscount 1678–1751Related quotes

This quote was already published in 1853 http://books.google.com/books?id=LM0QVhkWKrcC&pg=PA129&dq=%22two+eyes+are+geography+and+chronology.%22#v=onepage&q=%22two%20eyes%20are%20geography%20and%20chronology.%22&f=false, when Garfield was only 22.
Misattributed
“I have read somewhere that we often spend a lifetime searching for what we already have.”
Source: Slightly Married

Bętkowska, Teresa (August–September 2010). "Mistrz niszowej dyscypliny" http://www2.almamater.uj.edu.pl/126/17.pdf (PDF). Alma Mater (in Polish). Kraków: Jagiellonian University (126–127): pp. 41–46.
Source: Homage to the square' (1964), A conversation with Josef Albers' (1970), p. 459

Evil in Modern Thought: An alternative history of philosophy (2002)

Louis Althusser and Étienne Balibar, Reading Capital (1968), Part One: From Capital to Marx’s Philosophy
A - F, Louis Althusser