“When a man has put a limit on what he will do, he has put a limit on what he can do.”

As quoted in Shoptalk: learning to write with writers (1990), edited by Donald Morison Murray<!-- Cook Publishers -->
General sources
Context: A man is a fool not to put everything he has, at any given moment, into what he is creating. You're there now doing the thing on paper. You're not killing the goose, you're just producing an egg. So I don't worry about inspiration, or anything like that. It's a matter of just sitting down and working. I have never had the problem of a writing block. I've heard about it. I've felt reluctant to write on some days, for whole weeks, or sometimes even longer. I'd much rather go fishing, for example, or go sharpen pencils, or go swimming, or what not. But, later, coming back and reading what I have produced, I am unable to detect the difference between what came easily and when I had to sit down and say, 'Well, now it's writing time and now I'll write.' There's no difference on paper between the two.

The Architecture of Theories (1891)
Context: Of the fifty or hundred systems of philosophy that have been advanced at different times of the world's history, perhaps the larger number have been, not so much results of historical evolution, as happy thoughts which have accidently occurred to their authors. An idea which has been found interesting and fruitful has been adopted, developed, and forced to yield explanations of all sorts of phenomena. … The remaining systems of philosophy have been of the nature of reforms, sometimes amounting to radical revolutions, suggested by certain difficulties which have been found to beset systems previouslv in vogue; and such ought certainly to be in large part the motive of any new theory. … When a man is about to build a house, what a power of thinking he has to do, before he can safely break ground! With what pains he has to excogitate the precise wants that are to be supplied. What a study to ascertain the most available and suitable materials, to determine the mode of construction to which those materials are best adapted, and to answer a hundred such questions! Now without riding the metaphor too far, I think we may safely say that the studies preliminary to the construction of a great theory should be at least as deliberate and thorough as those that are preliminary to the building of a dwelling-house.

Source: Gone Girl

The Principles of Success in Literature (1865)

Un Art de Vivre (The Art of Living) (1939), The Art of Working
Context: A man's power and intelligence are limited. He who wants to do everything will never do anything. Only too well do we know those people of uncertain ability who say: "I could be a great musician."..."Business would be easy for me."..."I could surely make success in politics." We may be certain that they will always be amateur musicians, failures in business, and beaten politicians. Napoleon held that the art of war consisted of making oneself strongest at a certain point; in life we must choose a point of attack and concentrate our forces there. The choice of a career must not be left to chance.

A New Kind of Man
Song lyrics, A Sense of Wonder (1985)

Source: Arthur & George

Der Mensch kann tun was er will; er kann aber nicht wollen was er will.
Einstein paraphrasing Schopenhauer. Reportedly from On The Freedom Of The Will (1839), as translated in The Philosophy of American History: The Historical Field Theory (1945) by Morris Zucker, p. 531
Variant translations:
Man can do what he wants but he cannot want what he wants.
As quoted in The Motivated Brain: A Neurophysiological Analysis of Human Behavior (1991) by Pavel Vasilʹevich Simonov, p. 198
We can do what we wish, but we can only wish what we must.
As quoted by Einstein in "What Life Means to Einstein: An Interview by George Sylvester Viereck" The Saturday Evening Post (26 October 1929) p. 17. A scan of the article is available online here http://www.saturdayeveningpost.com/wp-content/uploads/satevepost/what_life_means_to_einstein.pdf (see p. 114).
Attributed
Source: Essays and Aphorisms

MSU WR Mark Dell, quoted here http://www.ncaa.com/sports/m-footbl/spec-rel/092108acj.html