Quotes Uncovered: Fools and Theory

A while back, I invited readers to submit quotations for which they wanted me to try to trace the origins, using The Yale Book of Quotations and more recent researches by me. Hundreds of people have responded via comments or e-mails. I am responding as best I can, a few per week.
psychohistorian asked:

“You can fool all the people half the time, and half the people all the time.” I seem to recall this being attributed to Lincoln. I also recall a professor who had gone through most or all of Lincoln’s works and never found it.

The Yale Book of Quotations, which attempts to trace all famous quotations to their earliest findable appearances, has the following:

“You can fool all of the people some of time; you can fool some of the people all of the time, but you can’t fool all the people all the time.”
Attributed to Abraham Lincoln in The New York Times, August 27, 1887. According to The Collected Works of Abraham Lincoln, ed. Roy P. Basler, “Tradition has come to attribute to the Clinton [Illinois] speeches [September 2, 1858] this “most famous” of Lincoln’s utterances. Basler indicates, however, that there is no evidence of this saying in Lincoln documents. P.T. Barnum has also been a putative source for the quotation.

Frederick Michael asked:

“In theory, there is no difference between theory and practice. In practice, there is.” “Predictions are difficult, especially when they involve the future.” They are falsely attributed to Yogi Berra and Niels Bohr, respectively.

The first quote is presumably an apocryphal Yogiism. For the second one, The YBQ says:

“It is difficult to predict, especially the future.” Attributed to Niels Bohr in Marc Kac, “Statistics” (1975). Kac states that this saying may have been “an old Danish proverb.” K.K. Steincke, Goodbye and Thanks (1948), quotes it as a pun used in the Danish Parliament in the late 1930’s.

Michael in Iowa City asked:

I’m very curious if the expression “hard-knock life” existed before the musical Annie.

A search of the vast ProQuest database of historical newspapers and magazines does not reveal any pre-Annie usages, so I would conjecture that it was probably not in use before the musical.
Do any readers have any other quotations whose origins they would like me to attempt to trace?