Respectfully Quoted: A Dictionary of Quotations. 1989.
Abraham Lincoln (180965)
We must not promise what we ought not, lest we be called on to perform what we cannot.
Attributed to ABRAHAM LINCOLN, speech delivered before the first Republican state convention of Illinois, Bloomington, Illinois, May 29, 1856.The Writings of Abraham Lincoln, ed. Arthur B. Lapsley, vol. 2, p. 249 (1905).
This version of the speech has been questioned because it was reconstructed by Henry C. Whitney, who made notes at the time but did not write it out until 1896. He did not claim that it was literally correct, only that he had followed the argument and that in many cases the sentences were as Lincoln spoke them. The only contemporary account of the so-called Lost Speech was a brief report in the Alton, Illinois, Weekly Courier, June 5, 1856, which does not contain this sentence.
Some historians believe the Whitney reconstruction is not worthy of serious consideration.The Collected Works of Abraham Lincoln, ed. Roy P. Basler, vol. 2, p. 341 (1953).