Full Idea
One's degrees of belief in the members of an idealised partition should sum to 100%. That is all there is to the claim that degrees of belief should have the structure of probabilities.
Gist of Idea
A thing works like formal probability if all the options sum to 100%
Source
Dorothy Edgington (Conditionals (Stanf) [2006], 3.1)
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.11