“If only I had known, I should have become a watchmaker.”
According to The Quote Verifier (2006) by Ralph Keyes, Einstein never said any such thing. (According to p. 285 of the book's "source notes" Keyes checked New Statesman 16 April 1965, which is commonly cited as the source of this quote. Some other books claim it is from New Statesman 16 April 1955 and at least one has it as 1945, but a Google Books search http://books.google.com/advanced_book_search?num=10&q=einstein+watchmaker+%22new+statesman%22 with the date range restricted to 1900-1995 shows that all the earliest sources give it as 1965. This includes the earliest source located, The Penguin Dictionary of Modern Quotations from 1971, as can be verified by this search http://www.google.com/search?q=%22of+his+making+the+atom+bomb+possible.+quoted+in+new+statesman%2C+16+april+1965%22&btnG=Search+Books&tbm=bks&tbo=1.) Keyes notes that Einstein "did use similar words to make a very different point" when he wrote, in a 1954 letter to the editor at The Reporter magazine, "If I would be a young man again and had to decide how to make my living, I would not try to become a scientist or scholar or teacher. I would rather choose to be a plumber or a peddler in the hope to find that modest degree of independence still available under present circumstances."
Similarly, in Einstein and the Poet by William Hermanns, p. 86 http://books.google.com/books?id=QXCyjj6T5ZUC&lpg=PP1&pg=PA86#v=onepage&q&f=false, Einstein is quoted saying the following in a 1948 interview: "If I should be born again, I will become a cobbler and do my thinking in peace."
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Albert Einstein 702
German-born physicist and founder of the theory of relativi… 1879–1955Related quotes

Part 2: "The Habit of Truth", §5 (p. 35)
Science and Human Values (1956, 1965)

Molloy (1951)
Context: My life, my life, now I speak of it as of something over, now as of a joke which still goes on, and it is neither, for at the same time it is over and it goes on, and is there any tense for that? Watch wound and buried by the watchmaker, before he died, whose ruined works will one day speak of God, to the worms.

In a letter to Mr. Hartmann, c. 1865; as quoted in The Painters of Barbizon I – Millet, Rousseau and Diaz, by John W. Mollett, B.A.; publ. Sampton Low, Marston, Searle & Rivington, Limited, London, 1890, p. 81
Mr. Hartmann, who had bought this and two other pictures had waited for them fifteen years, at last became impatient, and wrote Rousseau: 'I shall only enjoy my pictures in my extreme old age, when I shall have become too blind to see them'. his biographer/friend Alfred Sensier wrote: this seemed to Mr. Hartmann 'as the reasoning of a troubled mind.' https://archive.org/details/souvenirssurthr00sensgoog?q=Theodore+Rousseau
1851 - 1867