Full Idea
It is necessary that if it is snowing then 2+2=4, but the fact that 2+2=4 does not obtain in virtue of the fact that it is snowing.
Gist of Idea
2+2=4 is necessary if it is snowing, but not true in virtue of the fact that it is snowing
Source
Kit Fine (Guide to Ground [2012], 1.01)
Book Reference
'Metaphysical Grounding', ed/tr. Correia,F/Schnieder,B [CUP 2012], p.38
A Reaction
Critics dislike 'in virtue of' (as vacuous), but I can't see how you can disagree with this obvervation of Fine's. You can hardly eliminate the word 'because' from English, or say p is because of some object. We demand the right to keep asking 'why?'!