Full Idea
The doctrine of double effect says (crudely) it is wrong to intentionally do a bad act for its good consequences, but it may be permissible to do a good act despite its foreseeable bad consequences. (..Shoot an innocent man to avoid his agonising death?)
Gist of Idea
Double Effect: no bad acts with good consequences, but possibly good acts despite bad consequences
Source
Jonathan Glover (Causing Death and Saving Lives [1977], §6)
Book Reference
Glover,Jonathan: 'Causing Death and Saving Lives' [Penguin 1982], p.86
A Reaction
Glover rejects this principle, because he is a utilitarian. The principle implies a doubtful sharp distinction between an act and its consequences. If you foresee bad consequences, why do you go ahead and do it? I doubt if there are purely good acts.