Emotivism says that moral judgments express our feelings. "X is good" is an emotional exclamation (not a truth claim), and means "Hurrah for X!"Emotivism basics
These questions are about Chapter 5 of Harry Gensler's Ethics: A Contemporary Introduction (Routledge: 1998 and 2011).
Emotivism says that moral judgments express positive or negative feelings. "X is good" means "Hurrah for X!" -- and "X is bad" means "Boo on X!"Logical positivism
Since moral judgments are exclamations, they can't be true or false. So there can't be moral truths or moral knowledge. We can reason about moral issues if we assume a system of norms. But we can't reason about basic moral principles.
Some emotivists base their view on logical positivism, which holds roughly that any genuine truth claim must be able to be tested by sense experience. Since moral judgments can't be tested by sense experience, they aren't genuine truth claims. So moral judgments only express feelings. Thus logical positivism leads to emotivism.Problems with emotivism
One problem with this argument is that logical positivism is self-refuting. It claims (roughly) "Any genuine truth claim must be able to be tested by sense experience." But this claim itself can't be tested by sense experience. So, by its own standard, logical positivism can't be a genuine truth claim.
Some argue that emotivism is better than the other approaches because it's simpler and explains more of the facts. They appeal to this principle, which is an important part of scientific method: "A view is better if it's simpler and explains more."Moderate emotivism
However, it isn't clear that emotivism explains morality adequately; by denying moral knowledge and moral truth, it seems to water down what morality is. Another problem is that moral judgments, instead of being essentially emotional, go from "very emotional" to "not very emotional." And moral judgments don't always translate plausibly into exclamations.
Emotivism also would seem to destroy the objectivity of scientific method -- since it would translate "A scientific view is better if it's simpler and explains more" into the exclamation "Hurrah for scientific views that are simpler and explain more!"
Emotivism claims that, in disputes about basic moral principles, we can't appeal to reason but only to emotion. This would seem to lead to propaganda wars in which each side, unable to resort to reason, simply tries to manipulate the feelings of the other side.
Moderate emotivism tries to add a stronger rationality component. While admitting that ethics is based on feelings, it insists that our feelings can be more or less rational to the extent that we're informed and impartial.