“All great and extraordinary actions come from the heart. There are seasons in human affairs, when qualities fit enough to conduct the common business of life, are feeble and useless; and when men must trust to emotion, for that safety which reason at such times can never give.”

Lecture XXVIL: On Habit - Part II, in “Elementary Sketches of Moral Philosophy”, delivered at The Royal Institution in the years 1804, 1805, and 1806 by the late Rev. Sydney Smith, M.A. (Spottiswoodes and Shaw (London: 1849)) http://www.archive.org/stream/elementarysketc03smitgoog#page/n438/mode/2up, p. 423-424
Another Variant: The history of the world shows us that men are not to be counted by their numbers, but by the fire and vigour of their passions; by their deep sense of injury; by their memory of past glory; by their eagerness for fresh fame; by their clear and steady resolution of ceasing to live, or of achieving a particular object, which, when it is once formed, strikes off a load of manacles and chains, and gives free space to all heavenly and heroic feelings. All great and extraordinary actions come from the heart. There are seasons in human affairs when qualities, fit enough to conduct the common business of life, are feeble and useless, when men must trust to emotion for that safety which reason at such times can never give. These are the feelings which led the ten thousand over the Carduchian mountains; these are the feelings by which a handful of Greeks broke in pieces the power of Persia; and in the fens of the Dutch, and on the mountains of the Swiss, defended the happiness and revenged the oppressions of man! God calls all the passions out in their keenness and vigour for the present safety of mankind, anger and revenge, and the heroic mind, and a readiness to suffer—all the secret strength, all the invisible array of the feelings—all that nature has reserved for the great scenes of the world. When the usual hopes and the common aids of man are all gone, nothing remains under God but those passions which have often proved the best ministers of His purpose and the surest protectors of the world.
Quoted by Theodore Roosevelt in his " Brotherhood and the Heroic Virtues http://www.theodore-roosevelt.com/images/research/txtspeeches/668.pdf" Address at the Veterans' Reunion, Burlington, Vermont, September 5, 1901 and published in Theodore Roosevelt's "The Strenuous Life: Essays and Addresses" by Dover Publications (April 23, 2009) in its Dover Thrift Editions (ISBN: 978-0486472294), p. 126-127
Elementary Sketches of Moral Philosophy (1849)
Context: The history of the world shows us that men are not to be counted by their numbers, but by the fire and vigour of their passions; by their deep sense of injury; by their memory of past glory; by their eagerness for fresh fame; by their clear and steady resolution of ceasing to live, or of achieving a particular object, which, when it is once formed, strikes off a load of manacles and chains, and gives free space to all heavenly and heroic feelings. All great and extraordinary actions come from the heart. There are seasons in human affairs, when qualities fit enough to conduct the common business of life, are feeble and useless; and when men must trust to emotion, for that safety which reason at such times can never give. These are the feelings which led the ten thousand over the Carduchian mountans; these are the feelings by which a handful of Greeks broke in pieces the power of Persia: they have, by turns, humbled Austria, reduced Spain; and in the fens of the Dutch, and on the mountains of the Swiss, defended the happiness, and revenged the oppressions, of man! God calls all the passions out in their keenness and vigour, for the present safety of mankind. Anger, and revenge, and the heroic mind, and a readiness to suffer;— all the secret strength, all the invisible array, of the feelings,— all that nature has reserved for the great scenes of the world. For the usual hopes, and the common aids of man, are all gone! Kings have perished, armies are subdued, nations mouldered away! Nothing remains, under God, but those passions which have often proved the best ministers of His vengeance, and the surest protectors of the world.