Full Idea
Accepting Ramsey's suggestion that 'if' and 'on the supposition that' come to the same thing, we get an equation which says ...you believe if A,B to the extent that you think that A&B is nearly as likely as A.
Gist of Idea
On the supposition view, believe if A,B to the extent that A&B is nearly as likely as A
Source
Dorothy Edgington (Conditionals (Stanf) [2006], 3.1)
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.11