note: this is a derivative work, primarily from (reference 1), see references at bottom
To find out, answer the following questions in a common sense manner, and decide if they justify the conclusion.
(not trick questions, answer using common sense)
Q. Who are we to decide that one view of morality is any better or worse than any other?
A. Did you think the dietitian knew more about which foods were healthy than the faith healer? Wasn't the dietitian more qualified? Did the dietitian have more knowledge relevant to healthy food? If you don't think so, how can there exist such a thing as a domain of expertise? How can knowledge matter at all? As with any domain of expertise, not all opinions matter. Opinions that can't stand up to critical scrutiny are not relevant.
Q. But David Hume says there is no observation about the way the world "is", that can tell us the way the world "ought" to be.
A. Did you think we should be more concerned with the well being of conscious creatures than the well being of rocks? The capacity of conscious to suffer and flourish gives us the "ought". We know many facts about what causes suffering and flourishing. Ought we introduce cholera into the water supply as a matter of public policy? If you think no, then you already derived an "ought" from an "is". In other words, it "is" a fact that introducing cholera into the water supply as a matter of public policy would unnecessarily increase the suffering of conscious creatures, therefore we "ought" not do it.
Q. But you can't precisely define "well being" and "conscious creatures", so there can't be a science of well being of conscious creatures.
A. Just because some questions can't be answered with exact precision, does not mean there are no discoverable facts, or that all answers are equally vacuous. Did you think that there exist medical sciences, even though medical science does not specifically define how fast a healthy person can run? Isn't there still an objective difference between a healthy person and a dead one? There is a difference between answers in principle and answers in practice. We may not know exactly how many fish are in the ocean, that is, we have no answer in practice. However, we do know that there is an answer in principle, whether we know it or not. We still know enough to exclude certain answers. For example, if someone says there are exactly 1000 fish in the ocean, we know that answer can be excluded. Maslow's hierarchy of needs has been around since 1943 (reference 2), and it defines many levels of well being (physiological, safety, love/belonging, esteem, self-actualization). Answers to consciousness can be found in neuroscience. We have already found the reptilian brain (which includes the body's vital functions), the limbic brain (which includes memory and emotions), and the neocortex (which includes language and abstract thought) (reference 3).