Hume's gap between fact and value
David Hume observed a logical puzzle: from statements of fact alone, you cannot derive a moral conclusion. Even if every human is naturally selfish (a factual claim), it does not follow that people ought to be selfish. The transition from 'is' to 'ought' requires an additional moral premise that goes beyond pure description. No amount of scientific data about human psychology, biology, or behavior can by itself tell you what humans should do.
This distinction demolishes arguments like 'humans are naturally aggressive, therefore nations ought to go to war' or 'people are naturally competitive, therefore capitalism is the right system.' These sneak in an 'ought' that was never established. The naturalness of an impulse says nothing about its moral status. Rape is natural in some animal species; that fact does not make it morally acceptable.
Implications for moral reasoning
If you cannot derive 'ought' from 'is', then moral reasoning must begin with some irreducible values or commitments. You cannot argue someone into caring about suffering or respecting autonomy using facts alone. You can give them reasons to care (suffering is bad, autonomy matters), but those reasons presuppose values that cannot themselves be purely factual.
This is why moral disagreement is often frustratingly persistent. Two people can accept identical facts about an issue and still reach opposite moral conclusions because they start from different values. The is-ought distinction clarifies that moral progress requires not just better facts but also deliberation about values. Philosophy cannot eliminate this gap, only make the underlying values explicit so they can be examined.