In 1906 I discovered the Theory of Types. After this it only remained to write the book out. Whitehead's teaching work left him not enough leisure for this mechanical job. I worked at it from ten to twelve hours a day for about eight months in the year, from 1907 to 1910. The manuscript became more and more vast, and every time that I went out for a walk I used to be afraid that the house would catch fire and the manuscript get burnt up. It was not, of course, the sort of manuscript that could be typed, or even copied.
Source:The Autobiography of Bertrand Russell, v.１, chap. 6: Principia Mathematica, 1967
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