On Sunday 6 June 1841 in Geneva, towards the end of the Ruskin family's long Continental tour (1840-41), Ruskin wrote in his diary:
Several times about the same hour on Sunday mornings - a fit of self reproach has come upon me for my idling style of occupation at present - & I have formed resolutions to be always trying to get knowledge of some kind or other or bodily strength - or some real - available - continuing good rather than the mere amusement of the time. (Ruskin Library, Lancaster MS 2, fol. 221r, and Evans and Whitehouse, Diaries I, p.199)
Eighteen months later, on Sunday 11 December 1842, when Ruskin was halfway through writing Modern Painters I at Denmark Hill (see period of composition of Modern Painters I), he turned to this diary entry and added a significant note on the facing page:
Dec. 11th. 42. Very odd - exactly the same fit came on me in the same church next year - and was the origin of Turners work. (Ruskin Library, Lancaster MS 2, fol. 220v, and Evans and Whitehouse, Diaries I, p.199)
He refers here to Sunday 12 June 1842. This evidence makes it clear that Ruskin did not start work in earnest on Modern Painters I before June 1842. (Years later he added, further up this facing page: 'The most important entry in all the books!') The entry in Ruskin's diary for 12 June 1842 itself makes no reference to the 'same fit' coming over him, but there is nothing unusual in this: he wrote up personal matters of this kind only when he felt like it. (See long gestation period of Modern Painters I, renewed plans for 'Turner's work', Ruskin and religion.)