Three is a magic number. Three times works for spells. The witches in Macbeth are fascinated by three (and of course there are three of them). And the Christian God is threefold, though I doubt that Carroll would bring that in, not consciously anyhow.
The Rule of Three is an old arithmetic formula that would have been well known to Dodgson from his schooldays. Cocker's Arithmetick introduces its discussion of the Rule of Three with the problem, "If 4 Yards of Cloth cost 12 Shillings, what will 6 Yards cost at that Rate?" The Rule of Three gives the answer to this problem directly. Dickens knew Cocker's book. 'According to Cocker'was a variant title for 'Hard Times'.
They are what we called 'proportion sums' at my school. Primary school I think, before we discovered algebra.
Last edited by John Whitworth; 12-19-2015 at 04:22 AM.
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