The problem is an important one because people try to do this kind of thing all the time. Take an example: ‘Hitting people causes pain [is-statement] so you ought to avoid doing it unnecessarily [ought-statement].’ This argument isn’t valid. The truth of the premise doesn’t guarantee the truth of the conclusion. In order to guarantee the truth of the conclusion you need another premise to be true: ‘You ought to avoid causing unnecessary pain.’ But this is an ought-statement that isn’t supported by our original is-statement! If someone asks us why they ought to believe this ought-statement, we need to give them another good argument that’s based on is-statements. However, if you try this you’ll find it’s surprisingly difficult to do. You could perhaps point out that people don’t like suffering pain but, again, this is-statement alone can’t guarantee the truth of our ought-statement. To do that, we need another premise: ‘You ought to avoid doing things that people don’t like.’ But, this is just another ought-statement in need of support! Distressingly, there seem to be no facts about the world that entail any moral claims. There seem to be no is-statements that will justify our ought-statements.