The ideal observer theory (IOT) offers an account of the truth/objectivity of moral judgments in terms of the approval or disapproval of an ideal observer. The theory receives explicit treatment by Adam Smith and Henry Sidgwick; Roderick Firth is the most well-known proponent of the theory in the twentieth century.

There are two versions of the theory. On one account the IOT is an analysis of what (some, all) moral judgments mean: A judgment that some act (or event or state of character) is good may be analyzed in terms of that act being approved of by an ideal observer (IO); some act is wrong if it would be disapproved of by an IO. Most, but not all, such accounts conceive of the IO in hypothetical terms, leaving open the question of whether there actually is an IO. The...