Full Idea
Two obvious propositions of which one can be deduced from the other both become more certain than either in isolation; thus in a complicated deductive system, many parts of which are obvious, the total probability may become all but absolute certainty.
Gist of Idea
If one proposition is deduced from another, they are more certain together than alone
Source
Bertrand Russell (Regressive Method for Premises in Mathematics [1907], p.279)
Book Reference
Russell,Bertrand: 'Essays in Analysis', ed/tr. Lackey,Douglas [George Braziller 1973], p.279
A Reaction
Thagard picked this remark out, in support of his work on coherence.