Max Weber, in "Science as a Vocation":
Only by strict specialization can the scientific worker become fully conscious, for once and perhaps never again in his lifetime, that he has achieved something that will endure. A really definitive and good accomplishment is today always a specialized accomplishment. And whoever lacks the capacity to put on blinders, so to speak, and to come up to the idea that the fate of his soul depends upon whether or not he makes the correct conjecture at this passage of this manuscript may as well stay away from science. He will never have what one may call the 'personal experience' of science. Without this strange intoxication, ridiculed by every outsider; without this passion, this 'thousands of years must pass before you enter into life and thousands more wait in silence' — according to whether or not you succeed in making this conjecture; without this, you have no calling for science and you should do something else. For nothing is worthy of man as man unless he can pursue it with passionate devotion.
Why does one engage in doing something that in reality never comes, and never can come, to an end?
[To] set to work and meet the 'demands of the day,' in human relations as well as in our vocation … is plain and simple, if each finds and obeys the demon who holds the fibers of his very life.
from "Science as a Vocation," found in English translation online at http://mail.www.anthropos-lab.net/wp/wp-content/uploads/2011/12/Weber-Science-as-a-Vocation.pdf, (accessed 20121027). Original citation: Published as "Wissenschaft als Beruf," Gesammlte Aufsätze zur Wissenschaftslehre (Tübingen, 1922), pp. 524-55; originally a speech at Munich University, 1918, published in 1919 by Duncker & Humblodt, München. From H.H. Gerth and C. Wright Mills (Translated and edited), From Max Weber: Essays in Sociology, pp. 129-156, New York: Oxford University Press, 1946.