Full Idea
To look for a single general theory of how to decide the right thing to do is like looking for a single theory of how to decide what to believe.
Gist of Idea
There is no one theory of how to act (or what to believe)
Source
Thomas Nagel (The Fragmentation of Value [1977], p.135)
Book Reference
Nagel,Thomas: 'Mortal Questions' [CUP 1981], p.135
A Reaction
Depends on your level of generality. Values and virtues are general guides which should be brought to every action, with 'higher' values guiding choice of what is relevant.