Iirc, Hume's guillotine states essentially that a statement of what should be cannot be derived from any system of statements only about what is. That is to say, morality cannot be derived only from reality. Basically, you need to have a set of axioms (which in this context are called "ethical systems") in order to reduce questions of morality to questions of fact.

Apologies for any errors I am only half-awake.