Prescriptivism says that "You ought to do this" is a universalizable prescription (not a truth claim), and means "Do this and let everyone do the same in similar cases." We are to pick our moral principles by trying to be informed and imaginative, and then seeing what we can consistently hold.Universalizable prescriptions
These questions are about Chapter 6 of Harry Gensler's Ethics: A Contemporary Introduction (Routledge: 1998 and 2011).
Prescriptivism sees moral judgments as a type of prescription, or imperative. Moral judgments, like the simple imperative "Close the door," don't state facts and aren't true or false. Instead, they express our will, or our desires.Two logical rules for "ought"
Ought judgments are universalizable prescriptions. "You ought to do this" is equivalent to "Do this and let everyone do the same in similar cases." So moral beliefs express our desire that a kind of act be done in the present case and in all similar cases -- including ones where we imagine ourselves in someone else's place.
Prescriptivism shows how can we be both free and rational in forming our moral beliefs. Moral beliefs can be free because they express our desires and aren't provable from facts. They can be rational because the logic of "ought" leads to a method of moral reasoning that engages our rational powers to their limits.
Moral beliefs are subject to two basic logical rules:Golden rule consistency
U. To be logically consistent, we must make similar evaluations about similar cases.Rule U holds because moral judgments are universalizable: it's part of their meaning that they apply to similar cases. Rule P holds because moral judgments are prescriptions (imperatives), and thus express our will, or our desires, about how we and others are to live.
P. To be logically consistent, we must keep our moral beliefs in harmony with how we live and want others to live.
Prescriptivism's GR consistency condition, which follows from these two logical rules, claims that this combination is inconsistent:Problems with prescriptivism
* I believe that I ought to do something to another.This consistency condition is a more precise version of the traditional golden rule ("Treat others as you want to be treated"). We violate it if we think that we ought to do something to another but don't desire that this be done to us in the same situation.
* I don't desire that this be done to me in the same situation.
To think rationally about ethics, we need to be informed, imaginative, and consistent; the most important part of consistency is to follow the golden rule. This approach can show that Nazi moral beliefs are irrational -- since Nazis wouldn't be consistent in their moral beliefs if they knew the facts of the case and exercised their imagination.
Prescriptivism, while it has important insights, seems to rest on a questionable foundation. It says that ought judgments are universalizable prescriptions (or imperatives), and not truth claims. This leads it to deny the possibility of moral knowledge and moral truths -- which seems to conflict with how we approach ethics in our daily lives.