Full Idea
In mathematics, except in the earliest parts, the propositions from which a given proposition is deduced generally give the reason why we believe the given proposition.
Gist of Idea
The sources of a proof are the reasons why we believe its conclusion
Source
Bertrand Russell (Regressive Method for Premises in Mathematics [1907], p.273)
Book Reference
Russell,Bertrand: 'Essays in Analysis', ed/tr. Lackey,Douglas [George Braziller 1973], p.273