Full Idea
According to Jackson, in asserting 'If A,B' the speaker expresses his belief that A⊃B, and also indicates that this belief is 'robust' with respect to the antecedent A - the speaker would not abandon A⊃B if he were to learn that A.
Clarification
⊃ is the material conditional, so A⊃B means ¬AvB
Gist of Idea
'If A,B' affirms that A⊃B, and also that this wouldn't change if A were certain
Source
report of Frank Jackson (On Assertion and Indicative Conditionals [1979]) by Dorothy Edgington - Conditionals (Stanf) 4.2
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.22
A Reaction
The point is that you must not believe A⊃B solely on the dubious grounds of ¬A. This is 'to ensure an assertable conditional is fit for modus ponens' - that is, that you really will affirm B when you learn that A is true. Nice idea.
Related Idea
Idea 14289 There are some assertable conditionals one would reject if one learned the antecedent [Jackson, by Edgington]