As David Hume pointed out, knowing about the physical world never, by itself, tells you how to behave ethically. To behave ethically, you must not only know what is out there, but also have a value system that tells you what ought to be out there and what your place ought to be in making that happen. Knowing about the natural world is not enough to build a moral system.
The is-ought problem is a meta-ethical philosophical concept articulated by David Hume. Hume's argument states that prescriptive statements, also known as moral statements or "ought" statements, cannot be derived from purely descriptive ("is") statements. The implication of the concept: there is no way to justify morality based solely on observational evidence. This type of argument is a form of non sequitur in that the conclusion does not follow from the premises.
In A Treatise of Human Nature, Hume states:
"In every system of morality, which I have hitherto met with, I have always remarked, that the author proceeds for some time in the ordinary ways of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when all of a sudden I am surprised to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not. This change is imperceptible; but is however, of the last consequence. For as this ought, or ought not, expresses some new relation or affirmation, 'tis necessary that it should be observed and explained; and at the same time that a reason should be given; for what seems altogether inconceivable, how this new relation can be a deduction from others, which are entirely different from it. But as authors do not commonly use this precaution, I shall presume to recommend it to the readers; and am persuaded, that this small attention would subvert all the vulgar systems of morality, and let us see, that the distinction of vice and virtue is not founded merely on the relations of objects, nor is perceived by reason."
The is-ought problem is commonly interpreted to mean: we cannot infer what we morally ought to do from purely factual premises. We can't derive an "ought" from an "is." Further reading of Hume gives a different emphasis. Regarding willful murder, he says:
"The vice entirely escapes you as long as you consider the object. You will never find it until you turn your reflection into your own breast and find a sentiment of disapprobation that derives from you towards this action."
In other words, evil isn't a feature of willful murder but in a judgment arising in sentiment. However Hume criticizes those who mistake their own feelings about things like murder for intrinsic qualities like murder.