Full Idea
The law of contradiction must have been originally discovered by generalising from instances, though, once discovered, it was found to be quite as indubitable as the instances.
Gist of Idea
Non-contradiction was learned from instances, and then found to be indubitable
Source
Bertrand Russell (Regressive Method for Premises in Mathematics [1907], p.274)
Book Reference
Russell,Bertrand: 'Essays in Analysis', ed/tr. Lackey,Douglas [George Braziller 1973], p.274