In Ch. 4 (“Wrongness and Reasons”) of Thomas Scanlon’s WHAT WE OWE TO EACH OTHER, Scanlon introduces us to the basic idea of his “contractualist” theory of moral rightness and wrongness. Specifically:
an act is wrong if its performance in the circumstances would be disallowed by any set of principles for the general regulation of behavior that no one could reasonably reject as a basis for informed, unforced general agreement.(p. 153, WWO)
There are many elements here to unpack, in order to fully understand Scanlon’s view. But it is in a certain family of views of moral wrongness (or moral wrongness that is also the wronging of a person): what Derek Parfit calls “complaint theories” of moral wrongness. On this kind of view, roughly, an action is morally wrong just in case (and because) someone would have sufficient reason to complain about it being performed or publicly allowed (the action being, in this sense, unjustifiable to others).