A quick introduction to metaethics.

In science, we try to come up with the most general & simple rules possible which predict/resemble reality. In ethics, we try to come up with the simplest/fewest ethical rules and premises  possible which, as a system, predict/resemble our moral intuitions.

Example: In science, you could do without a theory of gravity and instead have a separate theory for every single object being pulled in a specific direction which is a function of the objects around it. But, a theory of gravity with a single law is simpler but gives the same predictions and is hence superior. In ethics, you could go through every single possible instance of murder and say that it is good or bad. Alternatively, you can come up with more general rules such as "Killing is bad unless done in self-defense/war/etc....". The general rules, provided the fit our intuitions, are better.

In science, the standard which we measure our theories by is an objective, external reality which is the same for everyone. Hence, a scientific theory or law is equally right or wrong for everyone. In ethics, morals do not exist in reality but only in our minds. More problematically, morals are different for every person. Hence ethical laws or theories cannot be objectively good or bad. A theory or rule can equate perfectly with one person's moral world and not at all with another's. How do we get around this? Simple. We select a certain set of intuitions, a certain ethical world, and judge our theories against that world. Usually, the ethical world we use is fairly close to that of most people in our societies. Hence, when we say a certain theory is good we mean it is probabilistically likely to be a good fit for the internal moral world of most people in our society. 

Another problem with ethics is that, unlike in physics, the subjective wold in which we live can be changed by our theories. A theory in physics does not change the rules of the universe. Gravity exists whether or not we believe in it. (Note: This may change if we develop technology sufficiently advanced to manipulate the laws of reality. Then, our beliefs would begin to shape the world ). On the other hand, a moral theory can change the moral world it seeks to capture. A strong law against torture linking to many other strong intuitions not relating to torture could well convince a person listening to it to abandon what pro-torture intuitions they had. It is as if  in physics making a strong theory could bend reality to fit your theory. How do we get around this? Right now, we don't because most philosophers are idiots stuck in the past. Instead of using the intuitions of other, which are unaffected by the philosophers theories, they use their own moral intuitions which are changed and hence unreliable. How should we get around this? 1: quarantine moral theories. Don't publish ethical works. Sequester philosophers for life. 2: Ask representative samples of the population all moral questions you can think of, record the results, beam them into the philosophers prison. Now the philosophers have a moral wold to judge their theories against which is not change by their theories. Problem: [inhuman/[prevents any [social good/moral progress] coming about from philosophers work.]]

That was meta-ethics. Enjoy.

note 1: 

• I make a distinction between laws and premises. premises I take to mean intuitions, by which I mean specific, situational moral judgement. i.e: person X does Y for reason Z in a certain situation. How good-bad is this on a 1 - 10 scale? Laws are rules stretching across many situations.
• I say the purpose of ethics is to find good laws. Goodness = simplicity * accuracy. Accuracy = extent to which the law aligns with the data, which for ethics is moral intuitions.
• Problem: People's internal moral worlds may not be composed of intuitions. Rather, they could be composed, at least partially, of laws which in turn give rise to intuitions. I don't know why this is a problem but something smells very wrong here.

Note 2: Other issues exist which I have glossed over.

• Moral intuitions are unstable.
• Moral intuitions are difficult to access.
• etc...